Electroactive polymer manufacturing

ABSTRACT

Described herein are transducers and their fabrication. The transducers convert between mechanical and electrical energy. Some transducers of the present invention include a pre-strained polymer. The pre-strain improves the conversion between electrical and mechanical energy. The present invention provides methods for fabricating electromechanical devices including one or more electroactive polymers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority under U.S.C.§120 from co-pending U.S. patent application Ser. No. 11/335,805, filedJan. 18, 2006, which is incorporated herein for all purposes; the Ser.No. 11/335,805 patent application is a continuation application andclaimed priority under U.S.C. §120 from U.S. patent application Ser. No.10/893,730, filed Jul. 16, 2004, now U.S. Pat. No. 7,049,732, issued May23, 2006, which is incorporated herein for all purposes; the Ser. No.10/893,730 patent application is a divisional application of and claimedpriority under U.S.C. §120 from U.S. patent application Ser. No.09/619,847, filed Jul. 20, 2000, now U.S. Pat. No. 6,812,624 B1, issuedNov. 2, 2004, which is incorporated herein for all purposes; the U.S.Pat. No. 6,812,624 B1 patent claimed priority under 35 U.S.C. § 119(e)from i) U.S. Provisional Patent Application No. 60/144,556 filed Jul.20, 1999, naming R. E. Pelrine et al. as inventors, and titled“High-speed Electrically Actuated Polymers and Method of Use”, which isincorporated by reference herein for all purposes, ii) U.S. ProvisionalPatent Application No. 60/153,329 filed Sep. 10, 1999, naming R. E.Pelrine et al. as inventors, and titled “Electrostrictive Polymers AsMicroactuators”, which is incorporated by reference herein for allpurposes, iii) U.S. Provisional Patent Application No. 60/161,325 filedOct. 25, 1999, naming R. E. Pelrine et al. as inventors, and titled“Artificial Muscle Microactuators”, which is incorporated by referenceherein for all purposes, iv) U.S. Provisional Patent Application No.60/181,404 filed Feb. 9, 2000, naming R. D. Kornbluh et al. asinventors, and titled “Field Actuated Elastomeric Polymers”, which isincorporated by reference herein for all purposes, v) U.S. ProvisionalPatent Application No. 60/187,809 filed Mar. 8, 2000, naming R. E.Pelrine et al. as inventors, and titled “Polymer Actuators andMaterials”, which is incorporated by reference herein for all purposes;vi) U.S. Provisional Patent Application No. 60/192,237 filed Mar. 27,2000, naming R. D. Kornbluh et al. as inventors, and titled “PolymerActuators and Materials II”, which is incorporated by reference hereinfor all purposes, and vii) U.S. Provisional Patent Application No.60/184,217 filed Feb. 23, 2000, naming R. E. Pelrine et al. asinventors, and titled “Electroelastomers and their use for PowerGeneration”, which is incorporated by reference herein for all purposes.

U.S. GOVERNMENT RIGHTS

This application was made in part with government support under contractnumber N00014-96-C-0026 awarded by the Office of Naval Research; thisapplication was also made in part with government support under contractnumber DAAG55-98-K-001 awarded by the United States Army Research Officeand Defense Advanced Research Project Agency. The government has certainrights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates generally to electroactive polymers thatconvert between electrical energy and mechanical energy. Moreparticularly, the present invention relates to polymers and their use asgenerators, sensors, in actuators and various applications. The presentinvention also relates to additives added to a polymer, laminatescomprising a transducer and methods of fabricating a transducer.

In many applications, it is desirable to convert between electricalenergy and mechanical energy. Exemplary applications requiringtranslation from electrical to mechanical energy include robotics,pumps, speakers, general automation, disk drives and prosthetic devices.These applications include one or more actuators that convert electricalenergy into mechanical work—on a macroscopic or microscopic level.Common electric actuator technologies, such as electromagnetic motorsand solenoids, are not suitable for many of these applications, e.g.,when the required device size is small (e.g., micro or mesoscalemachines). Exemplary applications requiring translation from mechanicalto electrical energy include mechanical property sensors and heel strikegenerators. These applications include one or more transducers thatconvert mechanical energy into electrical energy. Common electricgenerator technologies, such as electromagnetic generators, are also notsuitable for many of these applications, e.g., when the required devicesize is small (e.g., in a person's shoe). These technologies are alsonot ideal when a large number of devices must be integrated into asingle structure or under various performance conditions such as whenhigh power density output is required at relatively low frequencies.

Several ‘smart materials’ have been used to convert between electricaland mechanical energy with limited success. These smart materialsinclude piezoelectric ceramics, shape memory alloys and magnetostrictivematerials. However, each smart material has a number of limitations thatprevent its broad usage. Certain piezoelectric ceramics, such as leadzirconium titanate (PZT), have been used to convert electrical tomechanical energy. While having suitable efficiency for a fewapplications, these piezoelectric ceramics are typically limited to astrain below about 1.6 percent and are often not suitable forapplications requiring greater strains than this. In addition, the highdensity of these materials often eliminates them from applicationsrequiring low weight. Irradiated polyvinylidene difluoride (PVDF) is anelectroactive polymer reported to have a strain of up to 4 percent whenconverting from electrical to mechanical energy. Similar to thepiezoelectric ceramics, the PVDF is often not suitable for applicationsrequiring strains greater than 4 percent. Shape memory alloys, such asnitinol, are capable of large strains and force outputs. These shapememory alloys have been limited from broad use by unacceptable energyefficiency, poor response time and prohibitive cost.

In addition to the performance limitations of piezoelectric ceramics andirradiated PVDF, their fabrication often presents a barrier toacceptability. Single crystal piezoelectric ceramics must be grown athigh temperatures coupled with a very slow cooling down process.Irradiated PVDF must be exposed to an electron beam for processing. Boththese processes are expensive and complex and may limit acceptability ofthese materials.

In view of the foregoing, alternative devices that convert betweenelectrical and mechanical energy would be desirable.

SUMMARY

In one aspect, the present invention relates to polymers that convertbetween electrical and mechanical energy. When a voltage is applied toelectrodes contacting a pre-strained polymer, the polymer deflects. Thisdeflection may be used to do mechanical work. Similarly, when thepolymer deflects, an electric field is produced in the polymer. Thiselectric field may be used to produce electrical energy. Some polymersof the present invention include additives that improve conversionbetween electrical and mechanical energy. Other polymers of the presentinvention include laminate layers that improve conversion betweenelectrical and mechanical energy.

Some polymers of the present invention are pre-strained. The pre-strainimproves the mechanical response of an electroactive polymer relative toa non-strained polymer. The pre-strain may vary in different directionsof a polymer to vary response of the polymer to the applied voltage.

In one aspect, the present invention relates to generators and actuatorscomprising an electroactive polymer and mechanical coupling to convertbetween mechanical and electrical energy. Several generators andactuators include structures that improve the performance of anelectroactive polymer.

In another aspect, the present invention relates to compliant electrodesthat conform to the changing shape of a polymer. Many of the electrodesare capable of maintaining electrical communication at the highdeflections encountered with pre-strained polymers of the presentinvention. In some embodiments, electrode compliance may vary withdirection.

In yet another aspect, the present invention provides methods forfabricating electromechanical devices comprising one or moreelectroactive polymers. Additives that improve conversion betweenelectrical and mechanical energy may be added during fabrication.Polymers of the present invention may be made by casting, dipping, spincoating, spraying or other known processes for fabrication of thinpolymer layers.

In yet another aspect, the invention relates to a transducer forconverting between mechanical and electrical energy. The transducercomprising at least two electrodes and a polymer arranged in a mannerwhich causes a portion of the polymer to deflect in response to a changein electric field and/or to change in electric field in response todeflection. The transducer also comprising a layer laminated to at leasta portion of one of the polymer and the at least two electrodes, andmechanically coupled to the polymer and/or one of the at least twoelectrodes.

In another aspect, the invention relates to a method of fabricating atransducer comprising a polymer comprising an additive and one or moreelectrodes. The method comprising adding an additive to a polymer. Themethod also comprising fixing a portion of the polymer to a solidmember. The method further comprising forming the one or more electrodeson the polymer.

These and other features and advantages of the present invention will bedescribed in the following description of the invention and associatedfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a top perspective view of a transducer beforeand after application of a voltage in accordance with one embodiment ofthe present invention.

FIG. 1C illustrates a textured surface for an electroactive polymerhaving a wavelike profile.

FIG. 1D illustrates an electroactive polymer including a texturedsurface having random texturing.

FIG. 1E illustrates a cross-sectional side view of a diaphragmtransducer including an electroactive polymer before application of avoltage in accordance with one embodiment of the present invention.

FIG. 1F illustrates a cross-sectional view of the electroactive polymerdiaphragm of FIG. 1E after application of a voltage in accordance withone embodiment of the present invention.

FIGS. 2A and 2B illustrate a device for converting between electricalenergy and mechanical energy before and after actuation in accordancewith a specific embodiment of the present invention.

FIG. 2C illustrates a device for converting between electrical energyand mechanical energy including additional components to improvedeflection in accordance with a specific embodiment of the presentinvention.

FIGS. 2D and 2E illustrate a device for converting between electricalenergy and mechanical energy before and after actuation in accordancewith a specific embodiment of the present invention.

FIG. 2F illustrates a cross-sectional side view of a transducerincluding multiple polymer layers in accordance with one embodiment ofthe present invention.

FIG. 2G illustrates a stacked multilayer device as an example ofartificial muscle in accordance with one embodiment of the presentinvention.

FIG. 2H illustrates a device for converting between electrical energyand mechanical energy comprising an electroactive polymer diaphragm inaccordance with another embodiment of the present invention.

FIG. 2I illustrates an inchworm-type actuator including a rolledelectroactive polymer in accordance with one embodiment of the presentinvention.

FIG. 2J illustrates a device for converting between electrical energyand mechanical energy in one direction in accordance with anotherembodiment of the present invention.

FIG. 2K illustrates a device for converting between electrical energyand mechanical energy in accordance with another embodiment of thepresent invention.

FIG. 2L illustrates the device of FIG. 2K with a 90 degree bendingangle.

FIG. 2M illustrates a device for converting between electrical energyand mechanical energy including two polymer layers in accordance withanother embodiment of the present invention.

FIGS. 2N and 2O illustrate a device for converting between electricalenergy and mechanical energy in accordance with another embodiment ofthe present invention.

FIG. 3 illustrates a structured electrode that provides one-directionalcompliance according to a specific embodiment of the present invention.

FIG. 4 illustrates a pre-strained polymer comprising a structuredelectrode that is not directionally compliant according to a specificembodiment of the present invention.

FIG. 5 illustrates textured electrodes in accordance with one embodimentof the present invention.

FIG. 6 illustrates a two-stage cascaded pumping system including twodiaphragm device pumps in accordance with a specific embodiment of thepresent invention.

FIG. 7A illustrates a process flow for fabricating an electromechanicaldevice having at least one pre-strained polymer in accordance with oneembodiment of the present invention.

FIGS. 7B-F illustrate a process for fabricating an electromechanicaldevice having multiple polymer layers in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one skilled in the art, thatthe present invention may be practiced without some or all of thesespecific details. In other instances, well known process steps and/orstructures have not been described in detail in order to notunnecessarily obscure the present invention.

Overview

Electroactive polymers deflect when actuated by electrical energy. Inone embodiment, an electroactive polymer refers to a polymer that actsas an insulating dielectric between two electrodes and may deflect uponapplication of a voltage difference between the two electrodes. In oneaspect, the present invention relates to polymers that are pre-strainedto improve conversion between electrical and mechanical energy. Thepre-strain improves the mechanical response of an electroactive polymerrelative to a non-strained electroactive polymer. The improvedmechanical response enables greater mechanical work for an electroactivepolymer, e.g., larger deflections and actuation pressures. For example,linear strains of at least about 200 percent and area strains of atleast about 300 percent are possible with pre-strained polymers of thepresent invention. The pre-strain may vary in different directions of apolymer. Combining directional variability of the pre-strain, differentways to constrain a polymer, scalability of electroactive polymers toboth micro and macro levels, and different polymer orientations (e.g.,rolling or stacking individual polymer layers) permits a broad range ofactuators that convert electrical energy into mechanical work. Theseactuators find use in a wide range of applications.

For ease of understanding, the present invention is described and shownby focusing on a single direction of energy conversion. Morespecifically, the present invention focuses on converting electricalenergy into mechanical energy. However, in all the figures anddiscussions for the present invention, it is important to note that thepolymers and devices may convert between electrical energy andmechanical energy bi-directionally. Thus, any of the polymer materials,polymer configurations, transducers, devices and actuators describedherein are also a transducer for converting mechanical energy toelectrical energy (a generator) in the reverse direction. Similarly, anyof the exemplary electrodes described herein may be used with agenerator of the present invention. Typically, a generator includes apolymer arranged in a manner which causes a change in electric field inresponse to deflection of a portion of the polymer.

Thus, polymers and transducers of the present invention may be used asan actuator to convert from electrical to mechanical energy or agenerator to convert from mechanical to electrical energy. For atransducer having a substantially constant thickness, one mechanism fordifferentiating the performance of the transducer as being an actuatoror a generator is in the change in net area orthogonal to the thicknessduring use. For these transducers, when the net area of the transducerdecreases, the transducer is acting as a generator. Conversely, when thenet area of the transducer increases, the transducer is acting as anactuator.

As the electroactive polymers of the present invention may deflect atlinear strains of at least about 200 percent, electrodes attached to thepolymers should also deflect without compromising mechanical orelectrical performance. Correspondingly, in another aspect, the presentinvention relates to compliant electrodes that conform to the shape ofan electroactive polymer they are attached to. The electrodes arecapable of maintaining electrical communication even at the highdeflections encountered with pre-strained polymers of the presentinvention. By way of example, strains at least about 50 percent arecommon with electrodes of the present invention. In some embodiments,compliance provided by the electrodes may vary with direction.

As the pre-strained polymers are suitable for use in both the micro andmacro scales, in a wide variety of actuators and in a broad range ofapplications, fabrication processes used with the present invention varygreatly. In another aspect, the present invention provides methods forfabricating electromechanical devices including one or more pre-strainedpolymers. Pre-strain may be achieved by a number of techniques such asmechanically stretching an electroactive polymer and fixing the polymerto one or more solid members while it is stretched.

General Structure of Devices

FIGS. 1A and 1B illustrate a top perspective view of a transducer 100 inaccordance with one embodiment of the present invention. The transducer100 includes a polymer 102 for converting between electrical energy andmechanical energy. Top and bottom electrodes 104 and 106 are attached tothe electroactive polymer 102 on its top and bottom surfacesrespectively to provide a voltage difference across a portion of thepolymer 102. The polymer 102 deflects with a change in electric fieldprovided by the top and bottom electrodes 104 and 106. Deflection of thetransducer 100 in response to a change in electric field provided by theelectrodes 104 and 106 is referred to as actuation. As the polymer 102changes in size, the deflection may be used to produce mechanical work.

FIG. 1B illustrates a top perspective view of the transducer 100including deflection in response to a change in electric field.Generally speaking, deflection refers to any displacement, expansion,contraction, torsion, linear or area strain, or any other deformation ofa portion of the polymer 102. The change in electric field correspondingto the voltage difference produced by the electrodes 104 and 106produces mechanical pressure within the pre-strained polymer 102. Inthis case, the unlike electrical charges produced by the electrodes 104and 106 are attracted to each other and provide a compressive forcebetween the electrodes 104 and 106 and an expansion force on the polymer102 in planar directions 108 and 110, causing the polymer 102 tocompress between the electrodes 104 and 106 and stretch in the planardirections 108 and 110.

In some cases, the electrodes 104 and 106 cover a limited portion of thepolymer 102 relative to the total area of the polymer. This may done toprevent electrical breakdown around the edge of polymer 102 or toachieve customized deflections in certain portions of the polymer. Asthe term is used herein, an active region is defined as a portion of thepolymer material 102 having sufficient electrostatic force to enabledeflection of the portion. As will be described below, a polymer of thepresent invention may have multiple active regions. Polymer 102 materialoutside an active area may act as an external spring force on the activearea during deflection. More specifically, material outside the activearea may resist active area deflection by its contraction or expansion.Removal of the voltage difference and the induced charge causes thereverse effects.

The electrodes 104 and 106 are compliant and change shape with thepolymer 102. The configuration of the polymer 102 and the electrodes 104and 106 provides for increasing polymer 102 response with deflection.More specifically, as the transducer 100 deflects, compression of thepolymer 102 brings the opposite charges of the electrodes 104 and 106closer and stretching of the polymer 102 separates similar charges ineach electrode. In one embodiment, one of the electrodes 104 and 106 isground.

Generally speaking, the transducer 100 continues to deflect untilmechanical forces balance the electrostatic forces driving thedeflection. The mechanical forces include elastic restoring forces ofthe polymer 102 material, the compliance of the electrodes 104 and 106,and any external resistance provided by a device and/or load coupled tothe transducer 100. The resultant deflection of the transducer 100 as aresult of the applied voltage may also depend on a number of otherfactors such as the polymer 102 dielectric constant and the polymer 102size.

Electroactive polymers in accordance with the present invention arecapable of deflection in any direction. After application of the voltagebetween the electrodes 104 and 106, the electroactive polymer 102increases in size in both planar directions 108 and 110. In some cases,the electroactive polymer 102 is incompressible, e.g. has asubstantially constant volume under stress. In this case, the polymer102 decreases in thickness as a result of the expansion in the planardirections 108 and 110. It should be noted that the present invention isnot limited to incompressible polymers and deflection of the polymer 102may not conform to such a simple relationship.

Application of a relatively large voltage difference between theelectrodes 104 and 106 on the transducer 100 shown in FIG. 1A will causetransducer 100 to change to a thinner, larger area shape as shown inFIG. 1B. In this manner, the transducer 100 converts electrical energyto mechanical energy. The transducer 100 also converts mechanical energyto electrical energy.

FIGS. 1A and 1B may be used to show one manner in which the transducer100 converts mechanical to electrical energy. For example, if thetransducer 100 is mechanically stretched by external forces to athinner, larger area shape such as that shown in FIG. 1B, and arelatively small voltage difference is applied between electrodes 104and 106, when the external forces are removed the transducer 100 willcontract in area between the electrodes to a shape such as in FIG. 1A.Stretching the transducer generally refers to deflecting the transducerfrom its original resting position—typically to have a larger net areabetween the electrodes, e.g. in the plane defined by directions 108 and110 between the electrodes. The resting position refers to the positionof the transducer 100 having no external electrical or mechanical inputand may include any pre-strain in the polymer. Once the transducer 100is stretched, the relatively small voltage difference is provided suchthat the resulting electrostatic forces are insufficient to balance theelastic restoring forces of the stretch. The transducer 100 thereforecontracts, and it becomes thicker and has a smaller planar area in theplane defined by directions 108 and 110 (orthogonal to the thicknessbetween electrodes). When the polymer 102 becomes thicker, it separateselectrodes 104 and 106 and their corresponding unlike charges, thusraising the electrical energy of the charge. Further, when theelectrodes 104 and 106 contract to a smaller area, like charges withineach electrode compress, also raising the electrical energy of thecharge. Thus, with different charges on the electrodes 104 and 106,contraction from a shape such as that shown in FIG. 1B to one such asthat shown in FIG. 1A raises the electrical energy of the charge. Thatis, mechanical deflection is being turned into electrical energy and thetransducer 100 is acting as a generator.

In some cases, the transducer 100 may be described electrically as avariable capacitor. The capacitance decreases for the shape change goingfrom that shown in FIG. 1B to that shown in FIG. 1A. Typically, thevoltage difference between electrodes 104 and 106 will be raised bycontraction. This is normally the case, for example, if additionalcharge is not added or subtracted from the electrodes 104 and 106 duringthe contraction process. The increase in electrical energy, U, may beillustrated by the formula U=0.5 Q²/C, where Q is the amount of positivecharge on the positive electrode and C is the variable capacitance whichrelates to the intrinsic dielectric properties of polymer 102 and itsgeometry. If Q is fixed and C decreases, then the electrical energy Uincreases. The increase in electrical energy and voltage can berecovered or used in a suitable device or electronic circuit inelectrical communication with electrodes 104 and 106. In addition, thetransducer 100 may be mechanically coupled to a mechanical input thatdeflects the polymer and provides mechanical energy.

The transducer 100 will convert mechanical energy to electrical energywhen it contracts. Some or all of the charge and energy can be removedwhen the transducer 100 is fully contracted in the plane defined bydirections 108 and 110, or charge and energy can be removed duringcontraction. If the electric field pressure increases and reachesbalance with the elastic restoring stresses during contraction, thecontraction will stop before full contraction, and no further elasticmechanical energy will be converted to electrical energy. Removing someof the charge and stored electrical energy reduces the electrical fieldpressure, thereby allowing contraction to continue and furtherconverting more mechanical energy to electrical energy. The exactelectrical behavior of the transducer 100 when operating as a generatordepends on any electrical and mechanical loading as well as theintrinsic properties of the polymer 102 and electrodes 104 and 106.

The electroactive polymer 102 is pre-strained. Pre-strain of a polymermay be described in one or more directions as the change in dimension inthat direction after pre-straining relative to the dimension in thatdirection before pre-straining. The pre-strain may comprise elasticdeformation of the polymer 102 and be formed, for example, by stretchingthe polymer in tension and fixing one or more of the edges whilestretched. The pre-strain improves conversion between electrical andmechanical energy. In one embodiment, prestrain improves the dielectricstrength of the polymer. For the transducer 100, the pre-strain allowsthe electroactive polymer 102 to deflect more and provide greatermechanical work when converting electrical to mechanical energy. For agenerator, pre-strain allows more charge to be placed on the electrodes104 and 106, thereby resulting in more generated electrical energy, e.g.in a cycle of the transducer 100 deflection. In one embodiment, thepre-strain is elastic. After actuation, an elastically pre-strainedpolymer could, in principle, be unfixed and return to its originalstate. The pre-strain may be imposed at the boundaries using a rigidframe or may be implemented locally for a portion of the polymer.

In one embodiment, pre-strain is applied uniformly over a portion of thepolymer 102 to produce an isotropic pre-strained polymer. By way ofexample, an acrylic elastomeric polymer may be stretched by 200-400percent in both planar directions. In another embodiment, pre-strain isapplied unequally in different directions for a portion of the polymer102 to produce an anisotropic pre-strained polymer. In this case, thepolymer 102 may deflect greater in one direction than another whenactuated. While not wishing to be bound by theory, it is believed thatpre-straining a polymer in one direction may increase the stiffness ofthe polymer in the pre-strain direction. Correspondingly, the polymer isrelatively stiffer in the high pre-strain direction and more compliantin the low pre-strain direction and, upon actuation, the majority ofdeflection occurs in the low pre-strain direction. In one embodiment,the transducer 100 enhances deflection in the direction 108 byexploiting large pre-strain in the perpendicular direction 110. By wayof example, an acrylic elastomeric polymer used as the transducer 100may be stretched by 100 percent in the direction 108 and by 500 percentin the perpendicular direction 110. Construction of the transducer 100and geometric edge constraints may also affect directional deflection aswill be described below with respect to actuators.

Anisotropic prestrain may also improve the performance of a transducerto convert mechanical to electrical energy in a generator mode. Inaddition to increasing the dielectric breakdown strength of the polymerand allowing more charge to be placed on the polymer, high pre-strainmay improve mechanical to electrical coupling in the low pre-straindirection. That is, more of the mechanical input into the low pre-straindirection can be converted to electrical output, thus raising theefficiency of the generator.

The quantity of pre-strain for a polymer may be based on theelectroactive polymer and the desired performance of the polymer in anactuator or application. For some polymers of the present invention,pre-strain in one or more directions may range from −100 percent to 600percent. By way of example, for a VHB acrylic elastomer having isotropicpre-strain, pre-strains of at least about 100 percent, and preferablybetween about 200-400 percent, may be used in each direction. In oneembodiment, the polymer is pre-strained by a factor in the range ofabout 1.5 times to 50 times the original area. For an anisotropicacrylic pre-strained to enhance actuation in a compliant direction,pre-strains between about 400-500 percent may be used in the stiffeneddirection and pre-strains between about 20-200 percent may be used inthe compliant direction. In some cases, pre-strain may be added in onedirection such that a negative pre-strain occurs in another direction,e.g. 600 percent in one direction coupled with −100 percent in anorthogonal direction. In these cases, the net change in area due to thepre-strain is typically positive.

Pre-strain may affect other properties of the polymer 102. Largepre-strains may change the elastic properties of the polymer and bringit into a stiffer regime with lower viscoelastic losses. For somepolymers, pre-strain increases the electrical breakdown strength of thepolymer 102, which allows for higher electric fields to be used withinthe polymer—permitting higher actuation pressures and higherdeflections.

Linear strain and area strain may be used to describe the deflection ofa pre-strained polymer. As the term is used herein, linear strain of apre-strained polymer refers to the deflection per unit length along aline of deflection relative to the unactuated state. Maximum linearstrains (tensile or compressive) of at least about 50 percent are commonfor pre-strained polymers of the present invention. Of course, a polymermay deflect with a strain less than the maximum, and the strain may beadjusted by adjusting the applied voltage. For some pre-strainedpolymers, maximum linear strains of at least about 100 percent arecommon. For polymers such as VHB 4910 as produced by 3M Corporation ofSt. Paul, Minn., maximum linear strains in the range of 40 to 215percent are common. Area strain of an electroactive polymer refers tothe change in planar area, e.g. the change in the plane defined bydirections 108 and 110 in FIGS. 1A and 1B, per unit area of the polymerupon actuation relative to the unactuated state. Maximum area strains ofat least about 100 percent are possible for pre-strained polymers of thepresent invention. For some pre-strained polymers, maximum area strainsin the range of 70 to 330 percent are common.

Generally, after the polymer is pre-strained, it may be fixed to one ormore objects. Each object may be suitably stiff to maintain the level ofpre-strain desired in the polymer. The polymer may be fixed to the oneor more objects according to any conventional method known in the artsuch as a chemical adhesive, an adhesive layer or material, mechanicalattachment, etc.

Transducers and pre-strained polymers of the present invention are notlimited to any particular geometry or linear deflection. For example,the polymer and electrodes may be formed into any geometry or shapeincluding tubes and rolls, stretched polymers attached between multiplerigid structures, stretched polymers attached across a frame of anygeometry—including curved or complex geometries, across a frame havingone or more joints, etc. Deflection of a transducer according to thepresent invention includes linear expansion and compression in one ormore directions, bending, axial deflection when the polymer is rolled,deflection out of a hole provided in a substrate, etc. Deflection of atransducer may be affected by how the polymer is constrained by a frameor rigid structures attached to the polymer. In one embodiment, aflexible material that is stiffer in elongation than the polymer isattached to one side of a transducer induces bending when the polymer isactuated. In another embodiment, a transducer that deflects out of theplane is referred to as a diaphragm. A diaphragm device for convertingbetween electrical energy and mechanical energy will be described inmore detail with respect to FIGS. 1E and 1F.

Transducers (including methods of using them and methods of fabricatingthem) in accordance with the present invention are described in reportsavailable from the New Energy and Industrial Technology DevelopmentOrganization (NEDO) offices under the reference title “Annual ResearchProgress Report for R&D of Micromachine Technology (R&D of HighFunctional Maintenance System for Power Plant Facilities)” for 1999, the“Annual Research Progress Report for R&D of Micromachine Technology (R&Dof High Functional Maintenance System for Power Plant Facilities)” for1998, the “Annual Research Progress Report for R&D of MicromachineTechnology (R&D of High Functional Maintenance System for Power PlantFacilities)” for 1997, or the “Annual Research Progress Report for R&Dof Micromachine Technology (R&D of High Functional Maintenance Systemfor Power Plant Facilities)” for 1996, all of which are incorporatedherein for all purposes. NEDO has several offices in Japan in additionto other offices in the United Sates, Australia, France, Thailand andChina.

Electroactive polymers in accordance with one embodiment of the presentinvention may include a textured surface. FIG. 1C illustrates a texturedsurface 150 for an electroactive polymer 152 having a wavelike profile.The textured surface 150 allows the polymer 152 to deflect using bendingof surface waves 154. Bending of the surface waves 154 providesdirectional compliance in a direction 155 with less resistance than bulkstretching for a stiff electrode attached to the polymer 152 in thedirection 155. The textured surface 150 may be characterized by troughsand crests, for example, about 0.1 micrometer to 40 micrometers wide andabout 0.1 micrometers to 20 micrometers deep. In this case, the wavewidth and depth is substantially less than the thickness of the polymer.In a specific embodiment, the troughs and crests are approximately 10micrometers wide and six micrometers deep on a polymer layer with athickness of 200 micrometers.

In one embodiment, a thin layer of stiff material 156, such as anelectrode, is attached to the polymer 152 to provide the wavelikeprofile. During fabrication, the electroactive polymer is stretched morethan it can stretch when actuated, and the thin layer of stiff material156 is attached to the stretched polymer 152 surface. Subsequently, thepolymer 152 is relaxed and the structure buckles to provide the texturedsurface.

In general, a textured surface may comprise any non-uniform ornon-smooth surface topography that allows a polymer to deflect usingdeformation in the polymer surface. By way of example, FIG. 1Dillustrates an electroactive polymer 160 including a roughened surface161 having random texturing. The roughened surface 160 allows for planardeflection that is not directionally compliant. Advantageously,deformation in surface topography may allow deflection of a stiffelectrode with less resistance than bulk stretching or compression. Itshould be noted that deflection of a pre-strained polymer having atextured surface may comprise a combination of surface deformation andbulk stretching of the polymer.

Textured or non-uniform surfaces for the polymer may also allow the useof a barrier layer and/or electrodes that rely on deformation of thetextured surfaces. The electrodes may include metals that bend accordingto the geometry of the polymer surface. The barrier layer may be used toblock charge in the event of local electrical breakdown in thepre-strained polymer material.

Materials suitable for use as a pre-strained polymer with the presentinvention may include any substantially insulating polymer or rubber (orcombination thereof) that deforms in response to an electrostatic forceor whose deformation results in a change in electric field. One suitablematerial is NuSil CF19-2186 as provided by NuSil Technology ofCarpenteria, Calif. More generally, exemplary materials suitable for useas a pre-strained polymer include silicone elastomers, acrylicelastomers, polyurethanes, thermoplastic elastomers, copolymerscomprising PVDF, pressure-sensitive adhesives, fluoroelastomers,polymers comprising silicone and acrylic moieties, and the like.Polymers comprising silicone and acrylic moieties may include copolymerscomprising silicone and acrylic moieties, polymer blends comprising asilicone elastomer and an acrylic elastomer, for example. Obviously,combinations of some of these materials may be used as the polymer intransducers of this invention.

One example of a suitable silicone elastomer is Dow Corning HS3 asprovided by Dow Corning of Wilmington, Del. One example of a suitablefluorosilicone is Dow Corning 730 as provided by Dow Corning ofWilmington, Del. One suitable example of a thermoplastic elastomer isstyrene butadiene styrene (SBS) block copolymer.

Some acrylics such as any acrylic in the 4900 VHB acrylic series asprovided by 3M Corp. of St. Paul, Minn. have properties suitable for useas the transducer polymer for this invention. Thus, in some embodiments,polymers suitable for use with the present invention may be made fromany monoethylenically unsaturated monomer (or combination of monomers)homopolymerizable to form a polymer having a glass transitiontemperature at most about 0 degrees Celsius. Preferred monoethylenicallyunsaturated monomers include isooctyl acrylate, 2-ethylhexyl acrylate,decyl acrylate, dodecyl acrylate, hexyl acrylate, isononyl acrylate,isooctyl methacrylate, and 2-ethylhexyl methacrylate. Any of themonomers may also include one or more halogens such as fluorine.

One example of a suitable copolymer includes both silicone and acrylicelastomer moieties. In some case, materials suitable for use with thepresent invention may contain combinations of one or more of the abovelisted materials. For example, one suitable polymer is a blend includinga silicone elastomer and an acrylic elastomer.

In many cases, materials used in accordance with the present inventionare commercially available polymers. The commercially available polymersmay include, for example, any commercially available silicone elastomer,polyurethane, PVDF copolymer and adhesive elastomer. Using commerciallyavailable materials provides cost-effective alternatives for transducersand associated devices of the present invention. The use of commerciallyavailable materials may also simplify fabrication. In a specificembodiment, the commercially available polymer is a commerciallyavailable acrylic elastomer comprising mixtures of aliphatic acrylatethat are photocured during fabrication. The elasticity of the acrylicelastomer results from a combination of the branched aliphatic groupsand cross-linking between the acrylic polymer chains.

Materials used as a pre-strained polymer may be selected based on one ormore material properties such as a high electrical breakdown strength, alow modulus of elasticity-(for large or small deformations), a highdielectric constant, etc. In one embodiment, the polymer is selectedsuch that is has an elastic modulus at most about 100 MPa. In anotherembodiment, the polymer is selected such that is has a maximum actuationpressure between about 0.05 MPa and about 10 MPa, and preferably betweenabout 0.3 MPa and about 3 MPa.

In another embodiment, the polymer is selected such that is has adielectric constant between about 2 and about 20, and preferably betweenabout 2.5 and about 12. For some applications, an electroactive polymeris selected based on one or more application demands such as a widetemperature and/or humidity range, repeatability, accuracy, low creep,reliability and endurance. Often, halogenated polymers, such asfluorinated or chlorinated polymers, exhibit a higher dielectricconstant than the base polymer. In one example, a high dielectricpolyurethane may be made from partially fluorinated urethane monomers.

Electroactive polymers of the present invention may also include one ormore additives to improve various properties. Examples of suitableclasses of materials include plasticizers, antioxidants, and highdielectric constant particulates. Examples of suitable plasticizersinclude high molecular-weight hydrocarbon oils, high molecular-weighthydrocarbon greases, Pentalyne H, Piccovar® AP Hydrocarbon Resins, Admex760, Plastolein 9720, silicone oils, silicone greases, Floral 105,silicone elastomers, nonionic surfactants, and the like. Of course,combinations of these materials may be used. In one embodiment, theantioxidant is a nonvolatile solid antioxidant.

In one preferred embodiment, the additives improve the ability of thepolymer to convert between mechanical energy and electrical energy.Generally, the additive may improve any polymer property or parameterrelated to the ability of the parameter to convert between mechanicalenergy and electrical energy. Polymer material properties and parametersrelated to the ability of the polymer to convert between mechanicalenergy and electrical energy include, for example, the dielectricbreakdown strength, maximum strain, dielectric constant, elasticmodulus, properties associated with the visco-elastic performance,properties associated with creep, response time and actuation voltage.The addition of a plasticizer may, for example, improve the functioningof a transducer of this invention by reducing the elastic modulus of thepolymer and/or increasing the dielectric breakdown strength of thepolymer.

In one embodiment, an additive is included in a polymer to improve thedielectric breakdown strength of the polymer. Improving the dielectricbreakdown strength allows the use of larger electrically actuatedstrains for the polymer. By way of example, a plasticizing additive maybe added to a polymer to increase the dielectric breakdown strength ofthe polymer. Alternatively, a synthetic resin may be added to astyrene-butadiene-styrene block copolymer to improve the dialecticbreakdown strength of the copolymer. For example, pentalyn-H as producedby Hercules, Inc. of Wilmington, Del. was added to Kraton D2104 asproduced by Shell Chemical of Houston, Tex. to improve the dialecticbreakdown strength of the Kraton D2104. Further detail on thefabrication of polymers including addition of one or more additives isprovided below. In this case, the ratio of pentalyn-H added may rangefrom about 0 to 2:1 by weight. In another embodiment, an additive isincluded to increase the dielectric constant of a polymer. For example,high dielectric constant particulates such as fine ceramic powders maybe added to increase the dielectric constant of a commercially availablepolymer. Alternatively, polymers such as polyurethane may be partiallyfluorinated to increase the dielectric constant.

Alternatively, an additive may be included in a polymer to reduce theelastic modulus of the polymer. Reducing the elastic modulus enableslarger strains for the polymer. In a specific embodiment, mineral oilwas added to a solution of Kraton D to reduce the elastic modulus of thepolymer. In this case, the ratio of mineral oil added may range fromabout 0 to 2:1 by weight. Specific materials included to reduce theelastic modulus of an acrylic polymer of the present invention includeany acrylic acids, acrylic adhesives, acrylics including flexible sidegroups such as isooctyl groups and 2-ethylhexyl groups, or any copolymerof acrylic acid and isooctyl acrylate.

Multiple additives may be included in a polymer to improve performanceof one or more material properties. In one embodiment, mineral oil andpentalyn-H were both added to a solution of Kraton D2104 to increase thedielectric breakdown strength and to reduce the elastic modulus of thepolymer. Alternatively, for a commercially available silicone rubberwhose stiffness has been increased by fine carbon particles used toincrease the dielectric constant, the stiffness may be reduced by theaddition of a carbon or silver filled silicone grease.

An additive may also be included in a polymer to provide an additionalproperty for the transducer. The additional property is not necessarilyassociated with polymer performance in converting between mechanical andelectrical energy. By way of example, pentalyn-H may be added to KratonD2104 to provide an adhesive property to the polymer. In this case, theadditive also aids in conversion between mechanical and electricalenergy. In a specific embodiment, polymers comprising Kraton D2104,pentalyn-H, mineral oil and fabricated using butyl acetate provided anadhesive polymer and a maximum linear strain in the range of about 70 to200 percent.

Suitable actuation voltages for pre-strained polymers of the presentinvention may vary based on the electroactive polymer material and itsproperties (e.g. the dielectric constant) as well as the dimensions ofthe polymer (e.g. the thickness between electrodes). By way of example,actuation electric fields for the polymer 102 in FIG. 1A may range inmagnitude from about 0 V/m to about 440 MegaVolts/meter. Actuationvoltages in this range may produce a pressure in the range of about 0 Pato about 10 MPa. To achieve a transducer capable of higher forces, thethickness of the polymer may be increased. Alternatively, multiplepolymer layers may be implemented. Actuation voltages for a particularpolymer may be reduced by increasing the dielectric constant, decreasingpolymer thickness and decreasing the modulus of elasticity, for example.

Pre-strained polymers of the present invention may cover a wide range ofthicknesses. In one embodiment, polymer thickness may range betweenabout 1 micrometer and 2 millimeters. Typical thicknesses beforepre-strain include 50-225 micrometers for HS3, 25-75 micrometers forNuSil CF 19-2186, 50-1000 micrometers for SBS, and 100-1000 microns forany of the 3M VHB 4900 series acrylic polymers. Polymer thickness may bereduced by stretching the film in one or both planar directions. In manycases, pre-strained polymers of the present invention may be fabricatedand implemented as thin films. Thicknesses suitable for these thin filmsmay be below 50 micrometers.

Transducers for converting between mechanical and electrical energy ofthe present invention also encompass multilayer laminates. In oneembodiment, a multilayer laminate refers to a structure including one ormore layers in addition to a single electroactive polymer and itscorresponding electrodes. In one embodiment, a multilayer laminaterefers to a structure having a transducer including an electroactivepolymer and its corresponding electrodes, a layer laminated to at leastone of the electrode and the polymer, and the layer mechanically coupledto a portion of the transducer. Multilayer laminates may be referred toas either external or internal. For external multilayer laminates, theone or more additional layers are not between the electrodes. Forinternal multilayer laminates, the one or more additional layers arebetween the electrodes. For either external or internal layers, thelayers may be adhered using an adhesive or glue layer, for example.

Internal multilayer laminates may be used for a wide variety ofpurposes. A layer may also be included in an internal multilayerlaminate to improve any mechanical or electrical property of thetransducer, e.g., stiffness, electrical resistance, tear resistance,etc. Internal multilayer laminates may include a layer having a greaterdielectric breakdown strength. Internal multilayer laminates may includemultiple layers of compatible materials separated by conducting orsemiconducting layers (e.g. metallic or polymer layers) to increasebreakdown strength of the laminate transducer. Compatible materialsrefer to materials that comprise the same or substantially similarmaterial or have the same or substantially similar properties (e.g.mechanical and/or electrical). Internal laminates of compatiblematerials relative to the polymer may be used to compensate formanufacturing defects in the polymer and provide greater transduceruniformity. By way of example, a 100 micrometer thick, single layerpolymer may have a defect that may affect the entire 100 micrometerthickness. In this case, a laminate of ten layers each having athickness of 10 micrometers may be used such that any manufacturingdefects are localized to a 10 micrometer polymer—thus providing acomparable 100 micrometer thick laminate structure, but with greateruniformity and fault tolerance compared to the single layer polymer.Internal laminates of compatible materials relative to the polymer mayalso be used to prevent any runaway pull-in effect. Runaway pull-ineffect refers to when the electrostatic forces between electrodesgetting closer increases faster than the elastic resistive forces of thepolymer. In such cases, the transducer may become electromechanicallyunstable, leading to rapid local thinning and electrical breakdown. Aninternal layer may also be used to afford a layer of protection(electrical or mechanical) to another layer in the composite. In oneembodiment, an electrical barrier layer is mechanically coupled betweenan electrode and the polymer to minimize the effect of any localizedbreakdown in the polymer. Breakdown may be defined as the point at whichthe polymer cannot sustain the applied voltage. The barrier layer istypically thinner than the polymer and has a higher dielectric constantthan the polymer such that the voltage drop mainly occurs across thepolymer. It is often preferable that the barrier layer have a highdielectric breakdown strength.

External multilayer laminates may be used for a wide variety ofpurposes. In one embodiment, an external multilayer composite includes alayer to control stiffness, creep, to distribute load more uniformlyduring deflection, to increase tear resistance, or to prevent runawaypull effect. External laminates of compatible polymers includingelectrodes may be used to distribute load across each of the polymerlayers or increase polymer uniformity during deflection. A layer mayalso be included in an external laminate having a higher stiffness thanthe polymer, e.g., a material having a higher stiffness or a differentamount of pre-strain for a compatible material, to bias a diaphragm,pump or bending beam. In a generator mode, a stretched transducer maycontract and generate electrical energy as long as the electrical fieldstresses are lower than the elastic restoring stresses. In this case,adding a stiffening layer may allow the transducer to contract againstgreater field stresses, thereby increasing its energy output per stroke.An external layer may also be used to afford a layer of protection(electrical or mechanical) to another layer in the composite. In anotherspecific embodiment, an external composite includes a foam layer to biasa small pump or diaphragm. The foam layer may comprise an open pore foamthat allows fluids to move in and out of the foam. An external layerhaving a low stiffness may also be used for electric shielding withoutintroducing excessive mechanical energy loss.

In one embodiment, a composite is formed by rolling or folding a polymerto produce a transducer with high-density packaging. In order to avoiddetrimental electric fields in the vicinity of folds for laminatesincluding folded layers, electrodes may be patterned on the polymer suchthat any polymer in the vicinity of the folds does not have overlappingopposite electrodes. In addition, the polymer and electrodes may berolled or folded such that the outer exposed electrode or electrodeshave the same polarity. Fabrication may be performed such thatelectrodes of opposite polarity are separate by polymer. For example, arolled actuator can be made by rolling up two layers of polymer withelectrodes, or a single layer can be first folded, then rolled.Additionally, the outer exposed electrode may be grounded to increasesafety of the transducer. An external laminate outer skin layer may alsobe added to further increase safety.

Actuator and Generator Devices

The deflection of a pre-strained polymer can be used in a variety ofways to produce or receive mechanical energy. Generally speaking,electroactive polymers of the present invention may be implemented witha variety of actuators and generators—including conventional actuatorsand generators retrofitted with a pre-strained polymer and customactuators and generators specially designed for one or more pre-strainedpolymers. Conventional actuators and generators include extenders,bending beams, stacks, diaphragms, etc. Several different exemplarycustom actuators and generators in accordance with the present inventionwill now be discussed.

FIG. 1E illustrates a cross-sectional side view of a diaphragm device130 including a pre-strained polymer 131 before electrical actuation inaccordance with one embodiment of the present invention. Thepre-strained polymer 131 is attached to a frame 132. The frame 132includes a non-circular aperture 133 that allows deflection of thepolymer 131 perpendicular to the area of the non-circular aperture 133.The non-circular aperture 133 may be a rectangular slot, custom geometryaperture, etc. In some cases, a non-circular, elongated slot may beadvantageous for a diaphragm device compared to a circular hole. Forexample, thickness strain is more uniform for an elongated slot comparedto a hole. Non-uniform strains limit overall performance since theelectrical breakdown of a polymer is typically determined by thethinnest point. The diaphragm device 130 includes electrodes 134 and 136on either side of the polymer 131 to provide a voltage difference acrossa portion of the polymer 131.

In the voltage-off configuration of FIG. 1E, the polymer 131 isstretched and secured to the frame 132 with tension to achievepre-strain. Upon application of a suitable voltage to the electrodes 134and 136, the polymer film 131 expands away from the plane of the frame132 as illustrated in FIG. 1F. The electrodes 134 and 136 are compliantand change shape with the pre-strained polymer 131 as it deflects.

The diaphragm device 130 is capable of expansion in both directions awayfrom the plane. In one embodiment, the bottom side 141 of the polymer131 includes a bias pressure that influences the expansion of thepolymer film 131 to continually actuate upward in the direction ofarrows 143 (FIG. 1F). In another embodiment, a swelling agent such as asmall amount of silicone oil is applied to the bottom side 141 toinfluence the expansion of the polymer 131 in the direction of arrows143. The swelling agent causes slight permanent deflection in onedirection as determined during fabrication, e.g. by supplying a slightpressure on the bottom side 141 when the swelling agent is applied. Theswelling agent allows the diaphragm to continually actuate in a desireddirection without using a bias pressure.

The amount of expansion for the diaphragm device 130 will vary based ona number of factors including the polymer 131 material, the appliedvoltage, the amount of pre-strain, any bias pressure, compliance of theelectrodes 134 and 136, etc. In one embodiment, the polymer 131 iscapable of deflections to a height 137 at least about 50 percent of thehole diameter 139 and may take a hemispheric shape at large deflections.In this case, an angle 147 formed between the polymer 131 and the frame132 may be less than 90 degrees.

The diaphragm device 130 may also be used as a generator. In this case,a pressure, such as a fluid pressure, acts as mechanical input to thediaphragm on the bottom 141 to stretch the transducer (polymer 134 andelectrodes 134 and 136) in the vicinity of the aperture 133 as shown inFIG. 1F. A voltage difference is applied between electrodes 134 and 136while the transducer is stretched, and releasing the pressure allows thediaphragm to contract and increase the stored electrical energy on thetransducer.

Expansion in one direction of an electroactive polymer may inducecontractile stresses in a second direction such as due to Poissoneffects. This may reduce the mechanical output for a transducer thatprovides mechanical output in the second direction. Correspondingly,actuators of the present invention may be designed to constrain apolymer in the non-output direction. In some cases, actuators may bedesigned to improve mechanical output using deflection in the non-outputdirection.

One device which uses deflection in one planar direction to improveenergy conversion in the other planar direction is a bow device. FIGS.2A and 2B illustrate a bow device 200 for converting between electricalenergy and mechanical energy before and after electrical actuation inaccordance with a specific embodiment of the present invention. The bowdevice 200 is a planar mechanism comprising a flexible frame 202 whichprovides mechanical assistance to improve conversion between electricalenergy and mechanical energy for a polymer 206 attached to the frame202. The frame 202 includes six rigid members 204 connected at joints205. The members 204 and joints 205 provide mechanical assistance bycoupling polymer deflection in a planar direction 208 into mechanicaloutput in a perpendicular planar direction 210. More specifically, theframe 202 is arranged such that a small deflection of the polymer 206 inthe direction 208 improves displacement in the perpendicular planardirection 210. Attached to opposite (top and bottom) surfaces of thepolymer 206 are electrodes 207 (bottom electrode on bottom side ofpolymer 206 not shown) to provide a voltage difference across a portionof the polymer 206.

The polymer 206 is configured with different levels of pre-strain in itsorthogonal directions. More specifically, the electroactive polymer 206includes a high pre-strain in the planar direction 208, and little or nopre-strain in the perpendicular planar direction 210. This anisotropicpre-strain is arranged relative to the geometry of the frame 202. Morespecifically, upon actuation across electrodes 207, the polymercontracts in the high pre-strained direction 208. With the restrictedmotion of the frame 202 and the lever arm provided by the members 204,this contraction helps drive deflection in the perpendicular planardirection 210. Thus, even for a short deflection of the polymer 206 inthe high pre-strain direction 208, the frame 202 bows outward in thedirection 210. In this manner, a small contraction in the highpre-strain direction 210 becomes a larger expansion in the relativelylow pre-strain direction 208.

Using the anisotropic pre-strain and constraint provided by the frame202, the bow device 200 allows contraction in one direction to enhancemechanical deflection and electrical to mechanical conversion inanother. In other words, a load 211 (FIG. 2B) attached to the bow device200 is coupled to deflection of the polymer 206 in twodirections—direction 208 and 210. Thus, as a result of the differentialpre-strain of the polymer 206 and the geometry of the frame 202, the bowdevice 200 is able to provide a larger mechanical displacement andmechanical energy output than an electroactive polymer alone for commonelectrical input.

The bow device 200 may be configured based on the polymer 206. By way ofexample, the geometry of the frame 202 and dimensions of the polymer 206may be adapted based on the polymer 206 material. In a specificembodiment using HS3 silicone as the polymer 206, the polymer 206preferably has a ratio in directions 208 and 210 of 9:2 with pre-strainsabout 270 percent and −25 percent in the directions 208 and 210respectively. Using this arrangement, linear strains of at least about100 percent in direction 210 are possible.

The pre-strain in the polymer 206 and constraint provided by the frame202 may also allow the bow device 200 to utilize lower actuationvoltages for the pre-strained polymer 206 for a given deflection. As thebow device 200 has a lower effective modulus of elasticity in the lowpre-strained direction 210, the mechanical constraint provided by theframe 202 allows the bow device 200 to be actuated in the direction 210to a larger deflection with a lower voltage. In addition, the highpre-strain in the direction 208 increases the breakdown strength of thepolymer 206, permitting higher voltages and higher deflections for thebow device 200.

As mentioned earlier with respect FIG. 1A, when a polymer expands as aresult of electrostatic forces, it continues to expand until mechanicalforces balance the electrostatic pressure driving the expansion. Whenthe load 211 is attached to the bow device 200, mechanical effectsprovided by the load 211 will influence the force balance and deflectionof the polymer 206. For example, if the load 211 resists expansion ofthe bow device 200, then the polymer 206 may not expand as much as ifwere there no load.

In one embodiment, the bow device 200 may include additional componentsto provide mechanical assistance and enhance mechanical output. By wayof example, springs 220 as shown in FIG. 2C may be attached to the bowdevice 200 to enhance deflection in the direction 210. The springs loadthe bow device 200 such that the spring force exerted by the springsopposes resistance provided by an external load. In some cases, thesprings 220 provide increasing assistance for bow device 200 deflection.In one embodiment, spring elements may be built into the joints 205instead of the external springs 220 to enhance deflection of the bowdevice 200. In addition, pre-strain may be increased to enhancedeflection. The load may also be coupled to the rigid members 204 on topand bottom of the frame 202 rather than on the rigid members of the sideof the frame 202 (as shown in FIG. 2B). Since the top and bottom rigidmembers 204 contract towards each other when voltage is applied as shownin FIG. 2B, the bow actuator 200 provides an exemplary device contractsin the plane upon application of a voltage rather than expands.

When used as a generator, the bow device 200 improves conversion ofmechanical to electrical energy. Recall that a generator (as in FIGS. 1Aand 1B) of the present invention will convert mechanical energy toelectrical energy when it contracts. Also recall that if the electricfield pressure increases and reaches balance with the elastic restoringstresses during contraction, the contraction will stop and may diminishefficiency. The elastic energy per unit volume in a polymer is typicallyproportional to the elastic restoring stress or pressure, e.g. thestress applied at a boundary. One way to maximize the elastic energy fora given restoring stress or pressure is to use a lower modulus polymer.Lower modulus polymers may, however, generally have lower breakdownstrengths, and may negate the advantages of a low modulus. The bowdevice 200 is one way to maximize the elastic energy for a given netrestoring stress or pressure without using a lower modulus material.This is done using the frame 202 in conjunction with anisotropicpre-strains in directions 208 and 210 so that the net restoring pressureor force for a given strain is less than it would be for free boundaryconditions on the polymer 206. A high prestrain in direction 208supplies elastic energy via the frame 202 to assist in the expansion inthe direction 210. With regards to expansion in direction 210, thepolymer acts as though it has a low modulus, and a large amount ofelastic energy can be stored for a given input force or input stress atthe boundary. Since the contraction in direction 208 is small and thearea change from the small contraction is correspondingly small, theelectrical behavior due to changes in direction 208 are minimal comparedto the large electrical behavior (e.g. change in capacitance) due to thelarge strain changes in direction 210. The polymer 206 therefore behavesas if it were a very low modulus material that stretches substantiallyin one direction (direction 210), allowing the bow device 200 to converta relatively large amount of energy per unit volume of polymer perstroke at high efficiencies compared to other devices using the samepolymer 206.

The shape and constraint of the polymer may affect deflection. An aspectratio for an electroactive polymer is defined as the ratio of its lengthto width. If the aspect ratio is high (e.g., an aspect ratio of at leastabout 4:1) and the polymer is constrained along its length by rigidmembers, than the combination may result in a substantially onedimensional deflection in the width direction.

FIGS. 2D and 2E illustrate a linear motion device 230 for convertingbetween electrical energy and mechanical energy before and afteractuation in accordance with a specific embodiment of the presentinvention. The linear motion device 230 is a planar mechanism havingmechanical translation in one direction. The linear motion device 230comprises a polymer 231 having a length 233 substantially greater thanits width 234 (e.g., an aspect ratio at least about 4:1). The polymer231 is attached on opposite sides to stiff members 232 of a frame alongits length 233. The stiff members 232 have a greater stiffness than thepolymer 231. The geometric edge constraint provided by the stiff members232 substantially prevents displacement in a direction 236 along thepolymer length 233 and facilitates deflection almost exclusively in adirection 235. When the linear motion device 230 is implemented with apolymer 231 having anisotropic pre-strain, such as a higher pre-strainin the direction 236 than in the direction 235, then the polymer 231 isstiffer in the direction 236 than in the direction 235 and largedeflections in the direction 235 may result. By way of example, such anarrangement may produce linear strains of at least about 200 percent foracrylics having an anisotropic pre-strain.

A collection of electroactive polymers or actuators may be mechanicallylinked to form a larger actuator with a common output, e.g. force and/ordisplacement. By using a small electroactive polymer as a base unit in acollection, conversion of electric energy to mechanical energy may bescaled according to an application. By way of example, multiple linearmotion devices 230 may be combined in series in the direction 235 toform an actuator having a cumulative deflection of all the linear motiondevices in the series. When transducing electric energy into mechanicalenergy, electroactive polymers—either individually or mechanicallylinked in a collection—may be referred to as ‘artificial muscle’. Forpurposes herein, artificial muscle is defined as one or more transducersand/or actuators having a single output force and/or displacement.Artificial muscle may be implemented on a micro or macro level and maycomprise any one or more of the actuators described herein.

FIG. 2F illustrates cross-sectional side view of a multilayer device 240for converting between electrical energy and mechanical energy as anexample of artificial muscle in accordance with a specific embodiment ofthe present invention. The multilayer device 240 includes fourpre-strained polymers 241 arranged in parallel and each attached to arigid frame 242 such that they have the same deflection. Electrodes 243and 244 are deposited on opposite surfaces of each polymer 241 andprovide simultaneous electrostatic actuation to the four pre-strainedpolymers 241. The multilayer device 240 provides cumulative force outputof the individual polymer layers 241.

Combining individual polymer layers in parallel or in series has asimilar effect on transducers operated in a generator mode. In general,coupling layers in parallel increases the stiffness and maximum inputforce of the device without changing its maximum stroke, while combininglayers in series increases the maximum stroke without increasing themaximum input force. Thus, by combining layers in series and parallel, agenerator can be designed to match a specific input mechanical load.

In another embodiment, multiple electroactive polymer layers may be usedin place of one polymer to increase the force or pressure output of anactuator. For example, ten electroactive polymers may be layered toincrease the pressure output of the diaphragm device of FIG. 1E. FIG. 2Gillustrates such a stacked multilayer diaphragm device 245 forconverting between electrical energy and mechanical energy as anotherexample of artificial muscle in accordance with one embodiment of thepresent invention. The stacked multilayer device 245 includes threepolymer layers 246 layered upon each other and may be attached byadhesive layers 247. Within the adhesive layers 247 are electrodes 248and 249 that provide actuation to polymer layers 246. A relatively rigidplate 250 is attached to the outermost polymer layer and patterned toinclude holes 251 that allow deflection for the stacked multilayerdiaphragm device 245. By combining the polymer layers 246, the stackedmultilayer device 245 provides cumulative force output of the individualpolymer layers 246.

In addition to the linear motion device 230 of FIGS. 2D and 2E,electroactive polymers of the present invention may be included in avariety of devices that convert between electrical energy and mechanicalenergy. FIG. 2H illustrates a linear actuator 255 comprising anelectroactive polymer diaphragm 256 for converting between electricalenergy and mechanical energy in accordance with another embodiment ofthe present invention. In this case, an output shaft 257 is attached toa central portion of the diaphragm 256 that deflects in a non-circularaperture 258 of a frame 261. Upon actuation and removal of electrostaticenergy, the output shaft 257 translates as indicated by arrow 259. Thelinear actuator 255 may also include a compliant spring element 260 thathelps position the output shaft 257.

In one embodiment, the non-circular aperture 258 is an elongated slot.As noted previously, an elongated slot typically has more uniform strainthan a circular hole. In addition, the polymer diaphragm 256 has ahigher pre-strain in the long axis of the slot relative to thepre-strain in the perpendicular planar direction. By using relativelyhigh pre-strain in the long slot direction, and relatively lowpre-strain in the perpendicular planar direction, the displacement ofthe output shaft 257 can be increased relative to uniform pre-strainconfigurations.

In another embodiment, pre-strained polymers of the present inventionmay be rolled or folded into linear transducers and actuators thatdeflect axially while converting between electrical energy andmechanical energy. As fabrication of electroactive polymers is oftensimplest with fewer numbers of layers, rolled actuators provide anefficient manner of squeezing large layers of polymer into a compactshape. Rolled or folded transducers and actuators typically include twoor more layers of polymer. Rolled or folded actuators are applicablewherever linear actuators are used, such as robotic legs and fingers,high force grippers, and general-purpose linear actuators.

FIG. 2I illustrates an inchworm-type actuator 262 in accordance with aspecific embodiment of the present invention. The inchworm-type actuator262 includes two or more rolled pre-strained polymer layers withelectrodes 263 that deflect axially along its cylindrical axis. Theinchworm-type actuator 262 also includes electrostatic clamps 264 and265 for attaching and detaching to a metal surface 268. Theelectrostatic clamps 264 and 265 allow the total stroke for theinchworm-type actuator 262 to be increased compared to an actuatorwithout clamping. As the clamping force per unit weight for theelectrostatic clamps 264 and 265 is high, the force per unit weightadvantages of pre-strained polymers of the present invention arepreserved with the inchworm-type actuator 262. The electrostatic clamps264 and 265 are attached to the inchworm-type actuator at connectionregions 267. A body 266 of the inchworm-type actuator includes theconnection regions 267 and the polymer 263 and has a degree of freedomalong the axial direction of the rolled polymer 263 between theconnection regions 267. In one embodiment, the electrostatic clamps 264and 265 include an insulating adhesive 269 that prevents electricalshorting from the conductive electrostatic clamps 264 and 265 to themetal surface 268.

The inchworm-type actuator 262 moves upward in a six step process. Instep one, the inchworm-type actuator 262 is immobilized at itsrespective ends when both electrostatic clamps 264 and 265 are actuatedand the polymer 263 is relaxed. An electrostatic clamp is actuated byapplying a voltage difference between the clamp and the metal surface268. In step two, clamp 265 is released. Releasing one of the clamps 264and 265 allows its respective end of the inchworm-type actuator 262 tomove freely. In step three, the electroactive polymer 263 is actuatedand extends the inchworm-type actuator 262 upward. In step four, clamp265 is actuated and the inchworm-type actuator 262 is immobilized. Instep five, clamp 264 is released. In step six, the polymer 263 isrelaxed and the inchworm-type actuator 262 contracts. By cyclicallyrepeating steps one through six, the inchworm-type actuator 262 moves inthe upward direction. By switching clamps 264 and 265 in the above sixstep process, the inchworm-type actuator 262 moves in a reversedirection.

Although the inchworm-type actuator 262 has been described in terms ofactuation using a single electroactive polymer and two clamps, multiplesegment inchworm-type actuators using multiple electroactive polymersmay be implemented. Multiple segment inchworm-type actuators allow aninchworm-type actuator to increase in length without becoming thicker. Atwo-segment inchworm-type actuator would use two rolled polymers ratherthan one and three clamps rather than two. In general, an n-segmentinchworm-type actuator comprises n actuators between n+1 clamps.

FIG. 2J illustrates a stretched film device 270 for converting betweenelectrical energy and mechanical energy in accordance with anotherembodiment of the present invention. The stretched film device 270includes a rigid frame 271 having a hole 272. A pre-strained polymer 273is attached in tension to the frame 271 and spans the hole 272. A rigidbar 274 is attached to the center of the polymer 273 and providesexternal displacement corresponding to deflection of the polymer 273.Compliant electrode pairs 275 and 276 are patterned on both top andbottom surfaces of the polymer 273 on the left and right sidesrespectively of the rigid bar 274. When the electrode pair 275 isactuated, a portion of the polymer 273 between and in the vicinity ofthe top and bottom electrode pair 275 expands relative to the rest ofthe polymer 273 and the existing tension in the remainder of the polymer273 pulls the rigid bar 274 to move to the right. Conversely, when theelectrode pair 276 is actuated, a second portion of the polymer 273affected by the electrode pair 276 expands relative to the rest of thepolymer 273 and allows the rigid bar 274 to move to the left.Alternating actuation of the electrodes 275 and 276 provides aneffectively larger total stroke 279 for the rigid bar 274. One variationof this actuator includes adding anisotropic pre-strain to the polymersuch that the polymer has high pre-strain (and stiffness) in thedirection perpendicular to the rigid bar displacement. Another variationis to eliminate one of the electrode pairs. For the benefit ofsimplifying the design, this variation reduces the stroke 279 for thestretched film device 270. In this case, the portion of the polymer nolonger used by the removed electrode now responds passively like arestoring spring.

FIG. 2K illustrates a bending beam device 280 for converting betweenmechanical and electrical energy in accordance with another embodimentof the present invention. The bending beam device 280 includes a polymer281 fixed at one end by a rigid support 282 and attached to a flexiblethin material 283 such as polyimide or mylar using an adhesive layer,for example. The flexible thin material 283 has a modulus of elasticitygreater than the polymer 281. The difference in modulus of elasticityfor the top and bottom sides 286 and 287 of the bending beam device 280causes the bending beam device 280 to bend upon actuation. Electrodes284 and 285 are attached to the opposite sides of the polymer 281 toprovide electrical energy. The bending beam device 280 includes a freeend 288 having a single bending degree of freedom. Deflection of thefree end 288 may be measured by the difference in angle between the freeend 288 and the end fixed by the rigid support 282. FIG. 2L illustratesthe bending beam device 280 with a 90 degree bending angle.

The maximum bending angle for the bending beam device 280 will vary witha number of factors including the polymer material, the actuator length,the bending stiffness of the electrodes 284 and 285 and flexible thinmaterial 283, etc. For a bending beam device 280 comprising Dow CorningHS3 silicone, gold electrodes and an active area of 3.5 mm in length,bending angles over 225 degrees are attainable. For the bending beamdevice 280, as the length of the active area increases, increasedbending angles are attainable. Correspondingly, by extending the activelength of the above mentioned bending beam device to 5 mm allows for abending angle approaching 360 degrees.

In one embodiment, one of the electrodes may act as the flexible thinmaterial 283. Any thin metal, such as gold, having a low bendingstiffness and a high tensile stiffness may be suitable for an electrodeacting as the flexible thin material 283. In another embodiment, abarrier layer is attached between one of the electrodes 284 and 285 andthe polymer 281 to minimize the effect of any localized breakdown in thepolymer.

FIG. 2M illustrates a bending beam device 290 for converting betweenmechanical and electrical energy in accordance with another embodimentof the present invention. The bending beam device 290 includes top andbottom pre-strained polymers 291 and 292 fixed at one end by a rigidsupport 296. Each of the polymers 291 and 292 may be independentlyactuated. Independent actuation is achieved by separate electricalcontrol of top and bottom electrodes 293 and 294 attached to the top andbottom electroactive polymers 291 and 292, respectively. A commonelectrode 295 is situated between the top and bottom electroactivepolymers 291 and 292 and attached to both. The common electrode 295 maybe of sufficient stiffness to maintain the pre-strain on the polymerlayers 291 and 292 while still permitting extension and bending.

Actuating the top electroactive polymer 291 using the top pair ofelectrodes 293 and 295 causes the bending beam device 290 to benddownward. Actuating the bottom polymer 292 using the bottom pair ofelectrodes 294 and 295 causes the bending beam device 290 to bendupward. Thus, independent use of the top and bottom electroactivepolymers 291 and 292 allows the bending beam device 290 to be controlledalong a radial direction 297. When both top and bottom polymers 291 and292 are actuated simultaneously—and are of substantially similar sizeand material—the bending beam device 290 extends in length along thelinear direction 298. Combining the ability to control motion in theradial direction 297 and the linear direction 298, the bending beamdevice 290 becomes a two-degree-of-freedom actuator. Correspondingly,independent actuation and control of the top and bottom polymers 291 and292 allows a free end 299 of the bending beam device 290 to executecomplex motions such as circular or elliptical paths.

FIGS. 2N and 2O illustrate a device 300 for converting betweenelectrical energy and mechanical energy in accordance with anotherembodiment of the present invention. The device 300 includes a polymer302 arranged in a manner which causes a portion of the polymer todeflect in response to a change in electric field and/or arranged in amanner which causes a change in electric field in response to deflectionof the polymer. Electrodes 304 are attached to opposite surfaces (onlythe foremost electrode is shown) of the polymer 302 and cover asubstantial portion of the polymer 302. Two stiff members 308 and 310extend along opposite edges 312 and 314 of the polymer 302. Flexures 316and 318 are situated along the remaining edges of the polymer 302. Theflexures 316 and 318 improve conversion between electrical energy andmechanical energy for the device 300.

The flexures 316 and 318 couple polymer 302 deflection in one directioninto deflection in another direction. In one embodiment, each of theflexures 316 and 318 rest at an angle about 45 degrees in the plane ofthe polymer 302. Upon actuation of the polymer 302, expansion of thepolymer 302 in the direction 320 causes the stiff members 308 and 310 tomove apart, as shown in FIG. 2O. In addition, expansion of the polymer302 in the direction 322 causes the flexures 316 and 318 to straighten,and further separates the stiff members 308 and 310. In this manner, thedevice 300 couples expansion of the polymer 302 in both planardirections 320 and 322 into mechanical output in the direction 320.

In one embodiment, the polymer 302 is configured with different levelsof pre-strain in orthogonal directions 320 and 322. More specifically,the polymer 302 includes a higher pre-strain in the direction 320, andlittle or no pre-strain in the perpendicular planar direction 322. Thisanisotropic pre-strain is arranged relative to the geometry of theflexures 316 and 318.

One advantage of the device 300 is that the entire structure is planar.In addition to simplifying fabrication, the planar structure of thedevice 300 allows for easy mechanical coupling to produce multilayerdesigns. By way of example, the stiff members 308 and 310 may bemechanically coupled (e.g. glued or similarly fixed) to their respectivecounterparts of a second device 300 to provide two devices 300 inparallel in order to increase the force output over single device 300.Similarly, the stiff member 308 from one device may be attached to thestiff member 310 from a second device in order to provide multipledevices in series that increase the deflection output over a singledevice 300.

In addition to good performance of the device 300 as in actuator whichconverts electrical energy into mechanical energy, the device 300 isalso well-suited as a generator. For example, when a charge is placed onthe polymer 302 while it is stretched, contraction of the device 300converts mechanical energy to electrical energy. The electrical energymay then be harvested by a circuit in electrical communication with theelectrodes 304.

Performance

A transducer in accordance with the present invention converts betweenelectrical energy and mechanical energy. Transducer performance may becharacterized in terms of the transducer by itself, the performance ofthe transducer in an actuator, or the performance of the transducer in aspecific application (e.g., a number of transducers implemented in amotor). Pre-straining electroactive polymers in accordance with thepresent invention provides substantial improvements in transducerperformance.

Characterizing the performance of a transducer by itself usually relatesto the material properties of the polymer and electrodes. Performance ofan electroactive polymer may be described independent of polymer size byparameters such as strain, energy density, actuation pressure, actuationpressure density and efficiency. It should be noted that the performancecharacterization of pre-strained polymers and their respectivetransducers described below may vary for different electroactivepolymers and electrodes.

Pre-strained polymers of the present invention may have an effectivemodulus in the range of about 0.1 to about 100 MPa. Actuation pressureis defined as the change in force within a pre-strained polymer per unitcross-sectional area between actuated and unactuated states. In somecases, pre-strained polymers of the present invention may have anactuation pressure in the range of about 0 to about 100 MPa, and morepreferably in the range of about 0.1 to 10 MPa. Specific elastic energydensity—defined as the energy of deformation of a unit mass of thematerial in the transition between actuated and unactuated states—mayalso be used to describe an electroactive polymer where weight isimportant. Pre-strained polymers of the present invention may have aspecific elastic energy density above 3 J/g.

The performance of a pre-strained polymer may also be described,independent of polymer size, by efficiency. Electromechanical efficiencyis defined as the ratio of mechanical output energy to electrical inputenergy for an actuator mode of operation or, alternatively, of the ratioof electrical output energy to mechanical input energy for a generatormode of operation. Electromechanical efficiency greater than 80 percentis achievable with some pre-strained polymers of the present invention.The time for a pre-strained polymer to rise (or fall) to its maximum (orminimum) actuation pressure is referred to as its response time.Pre-strained polymer polymers in accordance with the present inventionmay accommodate a wide range of response times. Depending on the sizeand configuration of the polymer, response times may range from about0.01 milliseconds to 1 second, for example. A pre-strained polymerexcited at a high rate may also be characterized by an operationalfrequency. In one embodiment, maximum operational frequencies suitablefor use with the present invention may be in the range of about 100 Hzto 100 kHz. Operational frequencies in this range allow pre-strainedpolymers of the present invention to be used in various acousticapplications (e.g., speakers). In some embodiments, pre-strainedpolymers of the present invention may be operated at a resonantfrequency to improve mechanical output.

Performance of an actuator may be described by a performance parameterspecific to the actuator. By way of example, performance of an actuatorof a certain size and weight may be quantified by parameters such asstroke or displacement, force, actuator response time. Characterizingthe performance of a transducer in an application relates to how wellthe transducer is embodied in a particular application (e.g. inrobotics). Performance of a transducer in an application may bedescribed by a performance parameter specific to the application (e.g.,force/unit weight in robotic applications). Application specificparameters include stroke or displacement, force, actuator responsetime, frequency response, efficiency, etc. These parameters may dependon the size, mass and/or the design of the transducer and the particularapplication.

It should be noted that desirable material properties for anelectroactive polymer may vary with an actuator or application. Toproduce a large actuation pressures and large strain for an application,a pre-strained polymer may be implemented with one of a high dielectricstrength, a high dielectric constant, and a low modulus of elasticity.Additionally, a polymer may include one of a high-volume resistivity andlow mechanical damping for maximizing energy efficiency for anapplication.

Performance parameters for a transducer in a generator mode of operationare generally analogous to those of the actuator mode. Specific energydensity of a generator transducer is defined as the electrical energygenerated per stroke per unit mass of transducer (or polymer). Specificenergy densities for embodiments of the present invention are commonlyat least about 0.15 Joules per gram for the polymer and can be greaterthan 0.35 Joules per gram for some polymers.

Electrodes

As mentioned above, transducers of the present invention preferablyinclude one or more electrodes for actuating an electroactive polymer.Generally speaking, electrodes suitable for use with the presentinvention may be of any shape and material provided they are able tosupply or receive a suitable voltage, either constant or varying overtime, to or from an electroactive polymer. In one embodiment, theelectrodes adhere to a surface of the polymer. Electrodes adhering tothe polymer are preferably compliant and conform to the changing shapeof the polymer. The electrodes may be only applied to a portion of anelectroactive polymer and define an active area according to theirgeometry.

The compliant electrodes are capable of deflection in one or moredirections. Linear strain may be used to describe the deflection of acompliant electrode in one of these directions. As the term is usedherein, linear strain of a compliant electrode refers to the deflectionper unit length along a line of deflection. Maximum linear strains(tensile or compressive) of at least about 50 percent are possible forcompliant electrodes of the present invention. For some compliantelectrodes, maximum linear strains at least about 100 percent arecommon. Of course, an electrode may deflect with a strain less than themaximum. In one embodiment, the compliant electrode is a ‘structuredelectrode’ that comprises one or more regions of high conductivity andone or more regions of low conductivity.

FIG. 3 illustrates a top surface view of a structured electrode 501 thatprovides one-directional compliance in accordance with one embodiment ofthe present invention. The structured electrode 501 includes metaltraces 502 patterned in parallel lines over a charge distribution layer503—both of which cover an active area of a polymer (not shown). Themetal traces 502 and charge distribution layer 503 are applied toopposite surfaces of the polymer. Thus, the cross section, from top tobottom, of a transducer including structured electrodes 501 on oppositesurfaces is: top metal traces, top charge distribution layer, polymer,bottom charge distribution layer, bottom metal traces. Metal traces 502on either surface of the polymer act as electrodes for electroactivepolymer material between them. In another embodiment, the bottomelectrode may be a compliant, uniform electrode. The charge distributionlayer 503 facilitates distribution of charge between metal traces 502.Together, the high conductivity metal traces 502 quickly conduct chargeacross the active area to the low conductivity charge distribution layer503 which distributes the charge uniformly across the surface of thepolymer between the traces 502. The charge distribution layer 503 iscompliant. As a result, the structured electrode 501 allows deflectionin a compliant direction 506 perpendicular to the parallel metal traces502.

Actuation for the entire polymer may be achieved by extending the lengthof the parallel metal traces 502 across the length of the polymer and byimplementing a suitable number of traces 502 across the polymer width.In one embodiment, the metal traces 502 are spaced at intervals in theorder of 400 micrometers and have a thickness of about 20 to 100nanometers. The width of the traces is typically much less than thespacing. To increase the overall speed of response for the structuredelectrode 501, the distance between metal traces 502 may be reduced. Themetal traces 502 may comprise gold, silver, aluminum and many othermetals and relatively rigid conductive materials. In one embodiment,metal traces on opposite surfaces of an electroactive polymer are offsetfrom one another to improve charge distribution through the polymerlayer and prevent direct metal-to-metal electrical breakdowns.

Deflection of the parallel metal traces 502 along their axis greaterthan the elastic allowance of the metal trace material may lead todamage of the metal traces 502. To prevent damage in this manner, thepolymer may be constrained by a rigid structure that prevents deflectionof the polymer and the metal traces 502 along their axis. The rigidmembers 232 of the linear motion device of FIGS. 2D and 2E are suitablein this regard. In another embodiment, the metal traces 502 may beundulated slightly on the surface of the polymer 500. These undulationsadd compliance to the traces 502 along their axis and allow deflectionin this direction.

In general, the charge distribution layer 503 has a conductance greaterthan the electroactive polymer but less than the metal traces. Thenon-stringent conductivity requirements of the charge distribution layer503 allow a wide variety of materials to be used. By way of example, thecharge distribution layer may comprise carbon black, fluoroelastomerwith colloidal silver, a water-based latex rubber emulsion with a smallpercentage in mass loading of sodium iodide, and polyurethane withtetrathiafulavalene/tetracyanoquinodimethane (TTF/TCNQ) charge transfercomplex. These materials are able to form thin uniform layers with evencoverage and have a surface conductivity sufficient to conduct thecharge between metal traces 502 before substantial charge leaks into thesurroundings. In one embodiment, material for the charge distributionlayer 503 is selected based on the RC time constant of the polymer. Byway of example, surface resistivity for the charge distribution layer503 suitable for the present invention may be in the range of about10⁶-10¹¹ ohms. It should also be noted that in some embodiments, acharge distribution layer is not used and the metal traces 502 arepatterned directly on the polymer. In this case, air or another chemicalspecies on the polymer surface may be sufficient to carry charge betweenthe traces. This effect may be enhanced by increasing the surfaceconductivity through surface treatments such as plasma etching or ionimplantation. FIG. 4 illustrates a pre-strained polymer 510 underlying astructured electrode that is not directionally compliant according to aspecific embodiment of the present invention. The structured electrodeincludes metal traces 512 patterned directly on one surface of theelectroactive polymer 510 in evenly spaced parallel lines forming a‘zig-zag’ pattern. Two metal traces 512 on opposite surfaces of thepolymer act as electrodes for the electroactive polymer 510 materialbetween them. The ‘zig-zag’ pattern of the metal traces 512 allows forexpansion and contraction of the polymer and the structure electrode inmultiple directions 514 and 516.

Using an array of metal traces as described with respect to FIGS. 3 and4 permits the use of charge distribution layers having a lowerconductance. More specifically, as the spacing between metal tracesdecreases, the required conductance of the material between the tracesmay diminish. In this manner, it is possible to use materials that arenot normally considered conductive to be used as a charge distributionlayer. By way of example, polymers having a surface resistivity of 10¹⁰ohms may be used as an charge distribution layer in this manner. In aspecific embodiment, rubber was used as a charge distribution layer aspart of a structured electrode on a polymer layer having a thickness of25 micrometers and spacing between parallel metal traces of about 500micrometers. In addition to reducing the required conductance for acharge distribution layer, closely spaced metal traces also increase thespeed of actuation since the charge need only travel through the chargedistribution layer for a short distance between closely spaced metaltraces.

Although structured electrodes of the present invention have beendescribed in terms of two specific metal trace configurations;structured electrodes in accordance with the present invention may bepatterned in any suitable manner. As one skilled in the art willappreciate, various uniformly distributed metallic trace patterns mayprovide charge on the surface of a polymer while providing compliance inone or more directions. In some cases, a structured electrode may beattached to the surface of polymer in a non-uniform manner. As actuationof the polymer may be limited to an active region within suitableproximity of a pair of patterned metal traces, specialized active andnon-active regions for an electroactive polymer may be defined by custompatterning of the metal traces. These active and non-active regions maybe formed to custom geometries and high resolutions according toconventional metal trace deposition techniques. Extending this practiceacross the entire surface of an electroactive polymer, custom patternsfor structured electrodes comprising numerous custom geometry activeregions may result in specialized and non-uniform actuation of theelectroactive polymer according to the pattern of the structuredelectrodes.

Although the present invention has been discussed primarily in terms offlat electrodes, ‘textured electrodes’ comprising varying out of planedimensions may be used to provide a compliant electrode. FIG. 5illustrates exemplary textured electrodes 520 and 521 in accordance withone embodiment of the present invention. The textured electrodes 520 and521 are attached to opposite surfaces of an electroactive polymer 522such that deflection of the polymer 522 results in planar and non-planardeformation of the textured electrodes 520 and 521. The planar andnon-planar compliance of the electrodes 520 and 521 is provided by anundulating pattern which, upon planar and/or thickness deflection of thepolymer 522, provides directional compliance in a direction 526. Toprovide substantially uniform compliance for the textured electrodes 520and 521, the undulating pattern is implemented across the entire surfaceof the electroactive polymer in the direction 526. In one embodiment,the textured electrodes 520 and 521 are comprised of metal having athickness which allows bending without cracking of the metal to providecompliance. Typically, the textured electrode 520 is configured suchthat non-planar deflection of the electrodes 520 and 521 is much lessthan the thickness of the polymer 522 in order to provide asubstantially constant electric field to the polymer 522. Texturedelectrodes may provide compliance in more than one direction. In aspecific embodiment, a rough textured electrode provides compliance inorthogonal planar directions. The rough textured electrode may have atopography similar to the rough surface of FIG. 1D.

In one embodiment, compliant electrodes of the present inventioncomprise a conductive grease such as carbon grease or silver grease. Theconductive grease provides compliance in multiple directions. Particlesmay be added to increase the conductivity of the polymer. By way ofexample, carbon particles may be combined with a polymer binder such assilicone to produce a carbon grease that has low elasticity and highconductivity. Other materials may be blended into the conductive greaseto alter one or more material properties. Conductive greases inaccordance with the present invention are suitable for the deflection ofat least about 100 percent strain.

Compliant electrodes of the present invention may also include colloidalsuspensions. Colloidal suspensions contain submicrometer sizedparticles, such as graphite, silver and gold, in a liquid vehicle.Generally speaking, any colloidal suspension having sufficient loadingof conductive particles may be used as an electrode in accordance withthe present invention. In a specific embodiment, a conductive greaseincluding colloidal sized conductive particles is mixed with aconductive silicone including colloidal sized conductive particles in asilicone binder to produce a colloidal suspension that cures to form aconductive semi-solid. An advantage of colloidal suspensions is thatthey may be patterned on the surface of a polymer by spraying, dipcoating and other techniques that allow for a thin uniform coating of aliquid. To facilitate adhesion between the polymer and an electrode, abinder may be added to the electrode. By way of example, a water-basedlatex rubber or silicone may be added as a binder to a colloidalsuspension including graphite.

In another embodiment, compliant electrodes are achieved using a highaspect ratio conductive material such as carbon fibrils and carbonnanotubes. These high aspect ratio carbon materials may form highsurface conductivities in thin layers. High aspect ratio carbonmaterials may impart high conductivity to the surface of the polymer atrelatively low electrode thicknesses due to the high interconnectivityof the high aspect ratio carbon materials. By way of example,thicknesses for electrodes made with common forms of carbon that are nothigh-aspect ratio may be in the range of 5-50 micrometers whilethicknesses for electrodes made with carbon fibril or carbon nanotubeelectrodes may be less than 2-4 micrometers. Area expansions well over100 percent in multiple directions are suitable with carbon fibril andcarbon nanotube electrodes on acrylic and other polymers. High aspectratio carbon materials may include the use of a polymer binder toincrease adhesion with the electroactive polymer layer. Advantageously,the use of polymer binder allows a specific binder to be selected basedon adhesion with a particular electroactive polymer layer and based onelastic and mechanical properties of the polymer.

In one embodiment, high-aspect-ratio carbon electrodes may be fabricatedthin enough such that the opacity of the electrodes may be variedaccording to polymer deflection. By way of example, the electrodes maybe made thin enough such that the electrode changes from opaque tosemitransparent upon expansion. This ability to manipulate the opacityof the electrode may allow transducers of the present invention to beapplied to a number of various optical applications as will be describedbelow.

In another embodiment, mixtures of ionically conductive materials may beused for the compliant electrodes. This may include, for example, waterbased polymer materials such as glycerol or salt in gelatin,iodine-doped natural rubbers and water-based emulsions to which organicsalts such as potassium iodide are added. For hydrophobic electroactivepolymers that may not adhere well to a water based electrode, thesurface of the polymer may be pretreated by plasma etching or with afine powder such as graphite or carbon black to increase adherence.

Materials used for the electrodes of the present invention may varygreatly. Suitable materials used in an electrode may include graphite,carbon black, colloidal suspensions, thin metals including silver andgold, silver filled and carbon filled gels and polymers, ionically orelectronically conductive polymers. In a specific embodiment, anelectrode suitable for use with the present invention comprises 80percent carbon grease and 20 percent carbon black in a silicone rubberbinder such as Stockwell RTV60-CON as produced by Stockwell Rubber Co.Inc. of Philadelphia, Pa. The carbon grease is of the type such asCircuit Works 7200 as provided by ChemTronics Inc. of Kennesaw, Ga. Theconductive grease may also be mixed with an elastomer, such as siliconelastomer RTV 118 as produced by General Electric of Waterford, N.Y., toprovide a gel-like conductive grease.

It is understood that certain electrode materials may work well withparticular polymers and may not work as well for others. By way ofexample, carbon fibrils work well with acrylic elastomer polymers whilenot as well with silicone polymers. For most transducers, desirableproperties for the compliant electrode may include any one of a lowmodulus of elasticity, low mechanical damping, a low surfaceresistivity, uniform resistivity, chemical and environmental stability,chemical compatibility with the electroactive polymer, good adherence tothe electroactive polymer, and an ability to form smooth surfaces. Insome cases, it may be desirable for the electrode material to besuitable for precise patterning during fabrication. By way of example,the compliant electrode may be spray coated onto the polymer. In thiscase, material properties which benefit spray coating would bedesirable. In some cases, a transducer of the present invention mayimplement two different types of electrodes. By way of example, adiaphragm device of the present invention may have a structuredelectrode attached to its top surface and a high aspect ratio carbonmaterial deposited on the bottom side.

Electronic drivers are connected to the electrodes. The voltage providedto electroactive polymer will depend upon specifics of an application.In one embodiment, a transducer of the present invention is drivenelectrically by modulating an applied voltage about a DC bias voltage.Modulation about a bias voltage allows for improved sensitivity andlinearity of the transducer to the applied voltage. By way of example, atransducer used in an audio application may be driven by a signal of upto 200 to 1000 volts peak to peak on top of a bias voltage ranging fromabout 750 to 2000 volts DC.

Applications

As the present invention includes transducers that may be implemented inboth the micro and macro scales, and with a wide variety of actuatordesigns, the present invention finds use in a broad range ofapplications where conversion between electrical and mechanical energyis required. Provided below are several exemplary applications for someof the actuators described above. Broadly speaking, the transducers andactuators of the present invention may find use in any applicationrequiring conversion between electrical and mechanical energy. Theseapplications include robotics, sensors, motors, toys, micro-actuatorapplications, pumps, generators, etc.

As mentioned before, electroactive polymers, either individually ormechanically linked in a collection, may be referred to as artificialmuscle. The term artificial muscle in itself implies that theseactuators are well-suited for application to biologically inspiredrobots or biomedical applications where the duplication of muscle,mammalian or other, is desired. By way of example, applications such asprosthetic limbs, exoskeletons, and artificial hearts may benefit frompre-strained polymers of the present invention. The size scalability ofelectroactive polymers and the ability to use any number of transducersor polymer actuators in a collection allow artificial muscle inaccordance with the present invention to be used in a range inapplications greater than their biological counterparts. As transducersand actuators of the present invention have a performance range faroutside their biological counterparts, the present invention is notlimited to artificial muscle having a performance corresponding to realmuscle, and may indeed include applications requiring performanceoutside that of real muscle.

In one example of artificial muscle, a collection of linear motiondevices comprises two or more layers of pre-strained polymer sandwichedtogether and attached to two rigid plates at opposite edges of eachpolymer. Electrodes are sealed into the center between each of thepolymer layers. All of the linear motion devices in the collection maytake advantage of geometric constraints provided by the rigid plates andanisotropic pre-strain to restrict deformation of the polymer in theactuated direction. An advantage of the layered construction is that asmany electroactive polymer layers as required may be stacked in parallelin order to produce the desired force. Further, the stroke of thislinear motion device configuration may be increased by adding similarlinear motion devices in series.

In the micro domain, the pre-strained polymers may range in thicknessfrom several micrometers to several millimeters and preferably fromseveral micrometers to hundreds of micrometers. Micro pre-strainedpolymers are well-suited for applications such as inkjets, actuatedvalves, micropumps, inchworm-type actuators, pointing mirrors, soundgenerators, microclamps, and micro robotic applications. Micro roboticapplications may include micro robot legs, grippers, pointer actuatorsfor CCD cameras, wire feeders for micro welding and repair, clampingactuators to hold rigid positions, and ultrasonic actuators to transmitdata over measured distances. In another application, a diaphragm devicemay be implemented in an array of similar electroactive polymerdiaphragms in a planar configuration on a single surface. By way ofexample, an array may include sixty-two diaphragms with the diameter of150 micrometers each arranged in a planar configuration. In oneembodiment, the array of diaphragm devices may be formed on a siliconwafer. Diaphragm device arrays produced in this manner may include, forexample, from 5 to 10,000 or more diaphragms each having a diameter inthe range of about 60 to 150 micrometers. The array may be placed upongrid plates having suitably spaced holes for each diaphragm.

In the macro domain, each of the actuators described above may be wellsuited to its own set of applications. For example, the inchworm-typeactuator of FIG. 2I is suitable for use with small robots capable ofnavigating through pipes less than 2 cm in diameter. Other actuators arewell-suited, for example, with applications such as robotics, solenoids,sound generators, linear actuators, aerospace actuators, and generalautomation.

In another embodiment, a transducer of the present invention is used asan optical modulation device or an optical switch. The transducerincludes an electrode whose opacity varies with deflection. Atransparent or substantially translucent pre-strained polymer isattached to the opacity varying electrode and deflection of the polymeris used to modulate opacity of device. In the case of an optical switch,the opacity varying transducer interrupts a light source communicatingwith a light sensor. Thus, deflection of the transparent polymer causesthe opacity varying electrode to deflect and affect the light sensor. Ina specific embodiment, the opacity varying electrode includes carbonfibrils or carbon nanotubes that become less opaque as electrode areaincreases and the area fibril density decreases. In another specificembodiment, an optical modulation device comprised of an electroactivepolymer and an opacity varying electrode may be designed to preciselymodulate the amount of light transmitted through the device.

Diaphragm devices may be used as pumps, valves, etc. In one embodiment,a diaphragm device having a pre-strained polymer is suitable for use asa pump. Pumping action is created by repeatedly actuating the polymer.Electroactive polymer pumps in accordance with the present invention maybe implemented both in micro and macro scales. By way of example, thediaphragm may be used as a pump having a diameter in the range of about150 micrometers to about 2 centimeters. These pumps may include polymerstrains over 100 percent and may produce pressures of 20 kPa or more.

FIG. 6 illustrates a two-stage cascaded pumping system includingdiaphragm pumps 540 and 542 in accordance with a specific embodiment ofthe present invention. The diaphragm pumps 540 and 542 includepre-strained polymers 544 and 546 attached to frames 545 and 547. Thepolymers 544 and 546 deflect within holes 548 and 550 in the frames 545and 547 respectively in a direction perpendicular to the plane of theholes 548 and 550. The frames 545 and 547 along with the polymers 544and 546 define cavities 551 and 552. The pump 540 includes a plunger 553having a flexure spring 560 for providing a bias to the diaphragm 544towards the cavity 551.

A one-way valve 555 permits inlet of a fluid or gas into the cavity 551.A one-way valve 556 permits outlet of the fluid or gas out of the cavity551 into the cavity 552. In addition, a one-way valve 558 permits exitof the fluid or gas from the cavity 552. Upon actuation of the polymers544 and 546, the polymers deflect in turn to change the pressure withinthe cavities 551 and 552 respectively, thereby moving fluid or gas fromthe one-way valve 555 to the cavity 551, out the valve 556, into thecavity 552, and out the valve 558.

In the cascaded two-stage pumping system of FIG. 6, the diaphragm pump542 does not include a bias since the pressurized output from thediaphragm pump 540 biases the pump 542. In one embodiment, only thefirst pump in a cascaded series of diaphragm pumps uses a biaspressure—or any other mechanism for self priming. In some embodiments,diaphragm pumps provided in an array may include voltages provided byelectronic timing to increase pumping efficiency. In the embodimentshown in FIG. 6, polymers 544 and 546 are actuated simultaneously forbest performance. For other embodiments which may involve more diaphragmpumps in the cascade, the electronic timing for the different actuatorsis ideally set so that one pump contracts in cavity volume while thenext pump in the series (as determined by the one-way valves) expands.In a specific embodiment, the diaphragm pump 540 supplies gas at a rateof 40 ml/min and a pressure about 1 kPa while the diaphragm pump 542supplies gas at substantially the same flow rate but increases thepressure to 2.5 kPa.

Bending beam devices, such as those described with respect to FIGS.2K-2M, may be used in a variety of commercial and aerospace devices andapplications such as fans, electrical switches and relays, and lightscanners—on the micro and macro level. For bending beam actuators usedas light scanners, a reflective surface such as aluminized mylar may bebonded to the free end of a bending beam actuator. More specifically,light is reflected when the bending beam is actuated and light passeswhen the bending beam is at rest. The reflector may then be used toreflect incoming light and form a scanned beam to form an arc or lineaccording to the deflection of the actuator. Arrays of bending beamactuators may also be used for flat-panel displays, to control airflowover a surface, for low profile speakers and vibration suppressors, as“smart furs” for controlling heat transfer and/or light absorption on asurface, and may act as cilia in a coordinated manner to manipulateobjects.

Polymers and polymer films that are rolled into a tubular or multilayercylinder actuator may be implemented as a piston that expands axiallyupon actuation. Such an actuator is analogous to a hydraulic orpneumatic piston, and may be implemented in any device or applicationthat uses these traditional forms of linear deflection.

An electroactive polymer actuator may also operate at high speeds for avariety of applications including sound generators and acousticspeakers, inkjet printers, fast MEMS switches etc. In a specificembodiment, an electroactive polymer diaphragm is used as a lightscanner. More specifically, a mirror may be placed on a flexure thatpushes down on a 5 mm diameter electroactive polymer diaphragm toprovide a mirrored flexure. Good scanning of images at a scanning anglefrom about 10 to 30 degrees may be accomplished with voltages in therange of about 190 to 300 volts and frequencies in the range of about 30to 300 Hz. Much larger scanning angles, up to 90 degrees for example,may also be accommodated using voltages in the range of 400 to 500 V. Inaddition, higher frequencies may be used with a stiffer mirroredflexure.

Transducers of the present invention also find wide use as generatorsfor converting mechanical energy into electrical energy. In particular,generators of the present invention are well-suited for use as heelstrike generators. More specifically, one or more transducers of thepresent invention may be used in a shoe to harness mechanical energyproduced from walking into electrical energy. Typically, a generatorincludes a polymer arranged in a manner which causes a change inelectric field and stored electrical energy in response to deflection ofa portion of the polymer. A mechanical input, such as a heel-strike,expands the transducer in one or both planar directions parallel to thesurface of the electrodes, thus increasing the stored elastic mechanicalenergy of the transducer. If electrical charge is then placed on theelectrodes in the stretched state (or more charge is added in thestretched state) and the transducer is allowed to contract, thetransducer converts some or all of its elastic mechanical energy to agreater amount of stored electrical energy. The greater storedelectrical energy may then be recovered or harvested by circuitry inelectrical communication with the electrodes. Some portion of theharvested energy can then be recycled back to provide the initial inputelectrical charge on the next cycle of expansion-contraction. Generatorsapplications also include transducers coupled to conventional combustionengines to make fuel-driven electrical generators, hand-driven crankgenerators, wave-powered generators, wind-powered generators, and othertypes of generators where a mechanical input is available for stretchingthe transducer.

It should be noted that transducers of the present invention may beimplemented to have more than one functionality. In other words, atransducer may act as an actuator, a generator and a sensor in the samedesign.

Fabrication

As the pre-strained polymers may be implemented both in the micro andmacro scales, in a wide variety of actuator designs, with a wide rangeof materials, and in a broad range of applications, fabricationprocesses used with the present invention may vary greatly. In oneaspect, the present invention provides methods for fabricatingelectromechanical devices including one or more pre-strained polymers.

FIG. 7A illustrates a process flow 600 for fabricating anelectromechanical device having at least one electroactive polymer layerin accordance with one embodiment of the present invention. Processes inaccordance with the present invention may include up to severaladditional steps not described or illustrated here in order not toobscure the present invention. In some cases, fabrication processes ofthe present invention may include conventional materials and techniquessuch as commercially available polymers and techniques used infabrication of microelectronics and electronics technologies. Forexample, micro diaphragm devices may be produced in situ on siliconusing conventional techniques to form the holes and apply the polymerand electrodes.

The process flow 600 begins by receiving or fabricating a polymer (602).The polymer may be received or fabricated according to several methods.In one embodiment, the polymer is a commercially available product suchas a commercially available acrylic elastomer film. In anotherembodiment, the polymer is a film produced by one of casting, dipping,spin coating or spraying. In one embodiment, the polymer is producedwhile minimizing variations in thickness or any other defects that maycompromise the maximize electric field that can be applied across thepolymer and thus compromise performance.

Spin coating typically involves applying a polymer mixture on a rigidsubstrate and spinning to a desired thickness. The polymer mixture mayinclude the polymer, a curing agent and a volatile dispersant orsolvent. The amount of dispersant, the volatility of the dispersant, andthe spin speed may be altered to produce a desired polymer. By way ofexample, polyurethane films may be spin coated in a solution ofpolyurethane and tetrahydrofuran (THF) or cyclohexanone. In the case ofsilicon substrates, the polymer may be spin coated on an aluminizedplastic or a silicon carbide. The aluminum and silicon carbide form asacrificial layer that is subsequently removed by a suitable etchant.Films in the range of one micrometer thick may been produced by spincoating in this manner. Spin coating of polymer films, such as silicone,may be done on a smooth non-sticking plastic substrate, such aspolymethyl methacrylate or teflon. The polymer film may then be releasedby mechanically peeling or with the assistance of alcohol or othersuitable release agent. Spin coating is also suitable for producingthicker polymers in the range of 10-750 micrometers. In some cases, thepolymer mixture may be centrifuged prior to spin coating to removeunwanted materials such as fillers, particulates, impurities andpigments used in commercial polymers. To increase centrifuge efficacy orto improve thickness consistency, a polymer may be diluted in a solventto lower its viscosity; e.g. silicone may be disbursed in naptha.

The polymer may then be pre-strained in one or more directions (604). Inone embodiment, pre-strain is achieved by mechanically stretching apolymer in or more directions and fixing it to one or more solid members(e.g., rigid plates) while strained. Another technique for maintainingpre-strain includes the use of one or more stiffeners. The stiffenersare long rigid structures placed on a polymer while it is in apre-strained state, e.g. while it is stretched. The stiffeners maintainthe pre-strain along their axis. The stiffeners may be arranged inparallel or other configurations to achieve directional compliance ofthe transducer. It should be noted that the increased stiffness alongthe stiffener axis comprises the increased stiffness provided by thestiffener material as well as the increased stiffness of the polymer inthe pre-strain direction.

Surfaces on the pre-strained polymer may be textured. In one embodimentto provide texturing, a polymer is stretched more than it can stretchwhen actuated, and a thin layer of stiff material is deposited on thestretched polymer surface. For example, the stiff material may be apolymer that is cured while the electroactive polymer is stretched.After curing, the electroactive polymer is relaxed and the structurebuckles to provide the textured surface. The thickness of the stiffmaterial may be altered to provide texturing on any scale, includingsubmicrometer levels. In another embodiment, textured surfaces areproduced by reactive ion etching (RIE). By way of example, RIE may beperformed on a pre-strained polymer comprising silicon with an RIE gascomprising 90 percent carbon tetrafluoride and 10 percent oxygen to forma surface with wave troughs and crests 4 to 5 micrometers in depth.

One or more electrodes are then formed on the polymer (606). For thesilicone polymer altered by RIE mentioned above, a thin layer of goldmay be sputter deposited on the RIE textured surface to provide atextured electrode. In another embodiment, one or more graphiteelectrodes may be patterned and deposited using a stencil. Electrodescomprising conductive greases mixed with a conductive silicone may befabricated by dissolving the conductive grease and the uncuredconductive silicone in a solvent. The solution may then be sprayed onthe electroactive polymer material and may include a mask or stencil toachieve a particular pattern.

The metal traces of the structured electrodes of FIGS. 3 and 4 may bepatterned photolithographically on top of the polymer or chargedistribution layer. By way of example, a layer of gold is sputterdeposited before depositing a photoresist over the gold. The photoresistand gold may be patterned according to conventional photolithographictechniques, e.g. using a mask followed by one or more rinses to removethe photoresist. A charge distribution layer added between the polymerand the metal traces may be deposited by spin coating, for example.

In a specific embodiment, a structured electrode is formed on a polymerby sputter depositing gold for about 2 to 3 minutes (according to adesired thickness) at about 150 angstroms per minute. HPR 506photoresist as provided by Arch Chemicals, of Norwalk, Conn. is thenspin coated on the gold at about 500 to 1500 rpm for about 20 to 30seconds and then baked at about 90 degrees Celsius. A mask is thenapplied before exposing the photoresist to UV light and development toremove unmasked portions of the photoresist. The gold is then etchedaway and the film is rinsed. The remaining photoresist is removed byexposure to UV light, development and rinsing. The gold traces may thenbe stretched to enhance strain tolerance.

Textured electrodes of the present invention may also be patternedphotolithographically. In this case, a photoresist is deposited on apre-strained polymer and patterned using a mask. Plasma etching mayremove portions of the electroactive polymer not protected by the maskin a desired pattern. The mask may be subsequently removed by a suitablewet etch. The active surfaces of the polymer may then be covered withthe thin layer of gold deposited by sputtering, for example.

The transducer, comprising the one or more polymer layers andelectrodes, is then packaged according to an application (608).Packaging may also include assembly of multiple transducers mechanicallylinked or stacked as multiple layers. In addition, mechanical andelectrical connections to the transducers may be formed according to anapplication.

Fabrication of polymers may also include the addition of one or moreadditives. In the additives example described above, mineral oil wasadded to a solution of Kraton D2104 as produced by Shell Chemical ofHouston, Tex. in a solvent such as butyl acetate to increase thedielectric breakdown strength. In a specific example, the solutioncontained 14.3 percent weight mineral oil and 32.1 percent weight KratonD2104. The solution was then cast onto glass and heated in an oven at 95degrees Celsius to remove any residual solvent and produce theelectroactive polymer. The polymer was then stretched on a frame by 150percent by 150 percent. Carbon grease electrodes were then smeared onopposite surfaces of the polymer. This process produced a transducerhaving a maximum linear strain in the range of about 70 to 100 percent.

The present invention also provides alternative methods for fabricatingelectromechanical devices including multiple layers of pre-strainedpolymer. In one embodiment, a process for fabricating electromechanicaldevices begins by obtaining or fabricating a polymer layer. The polymeris then stretched to the desired pre-strain and attached to a firstrigid frame. Next electrodes are deposited onto both sides of thepolymer so as to define active areas and establish electricalconnections. The electrodes may be patterned by a variety of well-knowntechniques such as spray coating through a mask. If desired, a secondpolymer layer is then stretched on a second frame. Electrodes are thenpatterned on this second polymer layer. The second polymer layer is thencoupled to the first layer by stacking their respective frames. Layersof suitable compliant adhesives may be used to bond the two layers andelectrodes, if needed. The size of the frames is chosen so as not tointerfere with the polymer layers making intimate contact. Ifinterference is present, then it may be desirable to remove the secondframe, e.g., by cutting away the polymer layer around the periphery ofthe first frame. If desired, a third layer of polymer with electrodesmay be added in a manner similar to how the second layer was added tothe first. This procedure may be continued until a desired number oflayers is reached.

Rigid frames, rigid members or other electrical and mechanicalconnectors are then attached to the polymer layers, e.g., by gluing. Ifdesired, the polymer may then be removed from the first frame. In somecases, the first frame may serve as a structural part of the finalactuator or actuators. For example, the first frame may be an array ofholes to produce an array of diaphragm devices.

FIGS. 7B-F illustrate a second process for fabricating anelectromechanical device 640 having multiple layers of electroactivepolymer in accordance with another embodiment of the present invention.Processes in accordance with the present invention may include up toseveral additional steps not described or illustrated here in order notto obscure the present invention. The process begins by producing apre-strained polymer 622 on a suitable rigid substrate 624, e.g. by spincoating a polymer on a polymethyl methacrylate (PMMA) disk, stretchingthe polymer (FIG. 7B) and then attaching it to rigid substrate 624.After the polymer 622 is cured, electrodes 625 are patterned on theexposed side 626 of the polymer 622. A solid member 627 such as aflexible film including one of polyimide, mylar or acetate film is thendeposited onto the electroactive polymer 622 (FIG. 7C) with a suitableadhesive 628.

The rigid substrate 624 is then released from the electroactive polymer622 (FIG. 7D). A releasing agent such as isopropyl alcohol may be usedto facilitate the release. Electrodes 629 are then patterned on thepreviously unexposed side of the polymer 622. The assembly is thenbonded to another electroactive polymer layer 630 attached to a rigidsubstrate 631 (FIG. 7E). Polymer layers 622 and 630 may be bonded by anadhesive layer 632 comprising GE RTV 118 silicone, for example. Therigid substrate 631 is then released from the polymer 630 and electrodes633 are patterned on the available side 634 of the polymer 630. Ifadditional polymer layers are desired, the steps of adding a polymerlayer, removing the rigid substrate, and adding electrodes may berepeated to produce as many polymer layers as desired. Polymer layer 635has been added in this manner. To facilitate electrical communication toelectrodes in the inner layers of the device 640, a metal pin may bepushed through the structure to make contact with electrodes in eachlayer.

The solid member 627 may then be patterned or removed as needed toprovide the frame or mechanical connections required by the specificactuator type. In one embodiment, diaphragm devices may be formed bypatterning solid member 627 to form holes 636 which provide activeregions for the electromechanical device 640 using a suitable mask oretch technique (FIG. 7F). In another embodiment, if the active area isnot large and electrodes may be added to the active regions of thepolymers without damage, the solid member 627 may be patterned with theholes 636 prior to attachment to the polymer 622.

For the process of FIGS. 7B-F, the rigid substrate 624 is typicallyreleased from the electroactive polymer 622 by peeling the flexibleelectroactive polymer. Peeling is well-suited for fabricating devicescomprising electroactive polymers with a substantially flat profile. Inanother embodiment, sacrificial layers may be used between the polymeror electrodes and the rigid substrate to facilitate release. Thesacrificial layers allow the polymer, electrodes and attached assemblyto be released from a rigid substrate by etching away the sacrificiallayer. Metals comprising aluminum and silver are suitable for use as thesacrificial layers, for example. The use of metals allows thesacrificial layers to be etched away by liquids that do not affect thepolymer layers. Metal sacrificial layers may also be easily patternedwith various masking techniques to provide frames, connectors for otherstructural components for the electromechanical device 640. Thesacrificial layers may also be used to fabricate devices comprisingtransducers with non flat profiles, e.g. using rigid substrates shapedas tubes. For geometrically complex transducers, sacrificial layers maybe used in combination with dip coating to provide the complex geometry.

Although fabrication of pre-strained polymers has been briefly describedwith respect to a few specific examples, fabrication processes andtechniques of the present invention may vary accordingly for any theactuators or applications described above. For example, the process forfabricating a diaphragm device in accordance with a specific embodimentmay include spin coating a polymer on a substrate before a structuredelectrode is fabricated on the polymer. The polymer is then stretchedand rigid frames including one or more holes sized for the active areaof each diaphragm device are bonded to the pre-strained polymer,including any overlap portions of the structured electrode. In anotherembodiment, holes are etched into the substrate instead of using aseparate rigid frame, e.g. when the substrate is comprised of silicon.The substrate is then released from the polymer and an electrode isattached to the bottom side of the polymer.

CONCLUSION

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents thatfall within the scope of this invention which have been omitted forbrevity's sake. By way of example, although the present invention hasbeen described in terms of several numerous applied material electrodes,the present invention is not limited to these materials and in somecases may include air as an electrode. In addition, although the presentinvention has been described in terms of several preferred polymermaterials and geometries having particular performance ranges, thepresent invention is not limited to these materials and geometries andmay have performances outside the ranges listed. It is thereforeintended that the scope of the invention should be determined withreference to the appended claims.

1. A method of fabricating a device comprising an electroactive polymerand multiple electrodes, the method comprising: mechanically stretchinga polymer to form a pre-strained electroactive polymer, wherein thepre-strained electroactive polymer has an elastic modulus below 100 MPa;fixing a first portion of the pre-strained electroactive polymer to asolid member; forming a first electrode on a portion of a first surfaceof the pre-strained electroactive polymer; and forming a secondelectrode on a portion of a second surface of the pre-strainedelectroactive polymer.
 2. The method of claim 1 wherein the pre-strainedelectroactive polymer is elastically pre-strained.
 3. The method ofclaim 1 wherein the polymer is one of: a silicone elastomer, PVDFcopolymer, or adhesive elastomer.
 4. The method of claim 1 wherein thefirst portion of the pre-strained electroactive polymer includes an edgeportion of the electroactive polymer.
 5. The method of claim 1 whereinthe pre-strain is applied uniformly over a second portion of the polymerto produce an isotropic pre-strained electroactive polymer.
 6. Themethod of claim 1 wherein the pre-strain is applied unequally indifferent directions for a second portion of the polymer to produce ananisotropic pre-strained electroactive polymer.
 7. The method of claim 1wherein the pre-strain includes a strain between about 100 and about 500percent in each planar direction.
 8. The method of claim 1 whereinforming first electrode comprises spraying an electrode solution ontothe pre-strained polymer using a mask that defines the portion of thefirst surface.
 9. The method of claim 1 wherein the electrode solutionincludes carbon black, a binder and a solvent.
 10. The method of claim 1wherein fixing the portion of the pre-strained electroactive polymer tothe solid member includes attaching the solid member to the portionusing an adhesive.
 11. The method of claim 1 wherein the solid memberincludes one of: a rigid plastic or metal or a flexible plastic film ormetal film.
 12. A method of fabricating a device comprising anelectroactive polymer and multiple electrodes, the method comprising:mechanically stretching a polymer to form a pre-strained electroactivepolymer, wherein the pre-strained electroactive polymer has an elasticmodulus below 100 MPa; fixing a portion of the pre-strainedelectroactive polymer to a solid member; spraying an electrode solutiononto a first surface of the electroactive polymer using a first maskthat defines a pattern for a first electrode on the first surface of theelectroactive polymer; and spraying the electrode solution onto a secondsurface of the electroactive polymer using a second mask that defines apattern for a second electrode on the second surface of theelectroactive polymer.
 13. The method of claim 12 wherein the electrodesolution includes a conductive grease and a solvent.
 14. The method ofclaim 13 further comprising dissolving the conductive grease in thesolvent to form the electrode solution.
 15. The method of claim 13wherein the electrode solution also includes an uncured conductivesilicone or an uncured elastomer.
 16. The method of claim 12 whereinfixing the portion of the electroactive polymer to the solid member usesan adhesive.
 17. The method of claim 12 wherein the mechanical stretchincludes a strain greater than about 40 percent in a planar direction ofthe polymer.
 18. The method of claim 17 wherein the mechanical stretchincludes a strain greater than about 100 percent in a planar directionof the polymer.
 19. The method of claim 12 wherein the electroactivepolymer includes an additive.
 20. The method of claim 19 wherein theadditive improves at least one of polymer dielectric breakdown strength,maximum linear strain, dielectric constant, elastic modulus, responsetime, and actuation voltage.